Gunfire has been heard close to the government quarter in Kyiv after explosions hit the Ukrainian capital earlier in the morning, the Associated Press has reported.
The Ukrainian military also said that Russian spies and saboteurs had been spotted within the city, some 5 kilometers (3 miles) from the city center.
Russian tanks and troops have already reached the outskirts of the city after entering the country from Belarus to the north.
Social media videos showed what appeared to be Russian tanks in an area just north of Kyiv's city center.
Ukraine's military called on residents of Kyiv to track the movements of Russian forces in the city. It also told people to stay home and to make Molotov cocktails — a makeshift weapon that can be constructed from household items.
Radiation warnings at Chernobyl
Ukraine's nuclear agency warned of increased radiation levels coming out of Chernobyl on Friday, a day after Russian forces took control of the facility.
The infamous power plant has been decommissioned since 1986 when one of its nuclear reactors exploded, sending a radioactive cloud across Europe.
The agency reported detecting higher levels of gamma radiation which it attributed to a "disturbance of the topsoil due to the movement of a large amount of heavy military equipment through the exclusion zone and the release of contaminated radioactive dust into the air.''
Russia has denied the reports of an increase in radiation. It said troops had taken control of the area to protect it and prevent "provocations" from Kyiv.
Zelenskyy calls for eastern European defense assistance
Ukrainian President Volodymyr Zelenskyy said he had spoken with Polish counterpart Andrzej Duda on Friday to seek defense assistance from eastern European NATO members in the so-called Bucharest Nine organization.
"Together we have to put [Russia] at the negotiating table. We need (an) anti-war coalition," Zelenskyy wrote on Twitter.
The Bucharest Nine is an organization founded in 2015 by states that were either in the Soviet Union or within its sphere of influence in response to Russia's 2014 annexation of Crimea and later intervention in eastern Ukraine.
Its members, Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, and Slovakia, are scheduled to meet later on Friday.
Ukraine fears imminent attack on Kyiv
Ukrainian Deputy Defense Minister Hanna Malyar has warned that Russian forces are poised to enter areas just outside the capital, Kyiv, on Friday.
A government adviser, Anton Herashchenko, said the Russian troops planned to break through into the capital using tanks but that Ukrainian forces were ready to counter the attack with anti-tank missiles supplied by foreign allies.
Ukrainian President Volodymyr Zelenskyy had earlier said the government had information that "subversive groups" were moving on the city.
This comes after a night of reported air raids in the city that were said by a DW correspondent to have set some residential buildings on fire. Many Kyiv residents have taken shelter in underground metro stations.
The Ukrainian general staff of the armed forces said Ukrainian troops were firmly resisting what he called "Russian occupiers" in the Kyiv area.
Ukrainian airborne assault troops were reported in a statement to have stopped "overwhelming enemy forces" at the Teteriv River near the settlements of Dymer and Ivankiv, northwest of the capital.
The statement said a bridge over the river was destroyed.
"The hardest day will be today. The enemy's plan is to break through with tank columns from the side of Ivankiv and Chernihiv to Kyiv,'' Interior Ministry adviser Anton Gerashchenko said on Telegram.
Zelenskyy: Western sanctions 'not enough' to convince Russia
Ukrainian President Volodymyr Zelenskyy said on Friday it was clear that new Western sanctions imposed on Moscow were not enough to curb Russian aggression against his country.
Speaking after reported Russian missile attacks on the capital, Kyiv, he said the world was still just observing the events in Ukraine from a distance.
At the end of a televised speech, Zelenskyy switched to Russian and directed his words to Russian citizens, some of whom protested on Thursday against the war.
"To all the citizens of the Russian Federation who went out to protest, I want to say: We see you. This means you heard us. This means you are starting to believe us," Zelenskyy said.
"Fight for us, fight against the war," he added.
Hundreds of people were detained in Russia during the anti-war protests on Thursday.
DW correspondent: 'Kyiv is definitely the main target'
DW correspondent Mathias Bölinger, who is in Kyiv, spoke of "several air raids" in the night, with missiles landing on the city and planes flying over.
"We have seen burning parts of missiles or planes ... falling down on residential buildings; residential buildings have been on fire," he said, adding that there had been some civilian victims.
He said that although Ukraine had been "holding its defense quite well until now," Russian troops had made incursions into Ukrainian territory in many places.
"They are advancing from several directions, and Kyiv is definitely the main target," he said.
Explosions heard in Kyiv
The sound of explosions echoed through central Kyiv as Russian missiles fell on the Ukrainian capital early on Friday, news agencies have reported.
"Attacks on Kyiv with cruise and ballistic missiles have just resumed. I heard two powerful explosions," Ukrainian Interior Ministry advisor Anton Herashchenko said on Telegram.
He added that Ukrainian forces had downed an enemy aircraft over the capital in the early hours of Friday, which then crashed into a residential building and set it on fire.
Kyiv mayor Vitaly Klitschko said three people were injured, one being in critical condition, after missile debris hit a residential building.
He tweeted a photo showing a building with part of its wall torn down and firefighters present at the scene.
Attack on Zaporizhzhia border post
Authorities said intense fighting was under way on Friday morning in the city of Sumy in the country's northeast.
Meanwhile, Ukraine's border guard service said that a border post in the southeastern Zaporizhzhia region had been hit by a missile strike at 4.25 am local time (0225 GMT). Zaporizhzhia borders the Donetsk region to the east.
The service said that the attack had caused casualties.
Ukraine foreign minister: 'Horrific' airstrikes on Kyiv
Ukraine's Foreign Minister Dmytro Kuleba said Kyiv had suffered "horrific" Russian airstrikes. He went on to compare the attacks by Russian forces with World War II.
"Last time our capital experienced anHthing like this was in 1941 when it was attacked by Nazi Germany. Ukraine defeated that evil and will defeat this one," Kuleba said in a tweet.
Kuleba urged countries to adopt tough sanctions against Russia and to "several all ties" and "kick Russia out of everywhere."
"Stop Putin. Isolate Russia," he wrote.
Ukraine central bank bans payments to Russia, Belarus
The National Bank of Ukraine has banned payments to entities located in Russia and Belarus, the regulator said on Friday.
It has also banned operations involving Russian and Belarusian rubles.
Ukraine orders general mobilization
Ukrainian President Volodymyr Zelenskyy has signed a decree on the general mobilisation of the population in the wake of Russia's invasion.
Conscripts and reservists will be called up over the next 90 days to "ensure the defense of the state, maintaining combat and mobilization readiness," an entry on the Ukrainian presidency's website said.
"We have been left alone to defend our state," Zelenskyy said in a video address to the nation after midnight.
"Who is ready to fight alongside us? I don't see anyone. Who is ready to give Ukraine a guarantee of NATO membership? Everyone is afraid," he added.
The Ukrainian leader vowed to continue fighting, saying that "a new iron curtain" was falling between Russia and the West.
Ukraine's border guard said that males aged 18-60 are not allowed to leave the country in a statement posted on its Facebook account.
The restriction will last for the duration of the period of martial law in Ukraine.
Zelenskyy: Death toll over 100
Zelenskyy has announced that 137 citizens, including military personnel, had been killed and over 300 had been injured since Russia invaded
He called them "heroes" in the video address. Zelenskyy said that despite Russia's claim it is attacking only military targets, civilian sites also have been struck.
"They're killing people and turning peaceful cities into military targets. It's foul and will never be forgiven," he said.
Zelenskyy added that all the border guards on Zmiinyi island in Ukraine's southwestern Odesa region were killed on Thursday.
Earlier in the day, Russia took control Chernobyl, the site of a nuclear disaster in 1986 and where a decommissioned nuclear power plant and exclusion zone remain.
The White House said on Thursday that it was outraged at reports of hostages taken at the facilities at Chernobyl.
Summary of events in Ukraine on Thursday
Russia launched a full-scale invasion of Ukraine on Thursday. Its military attacked from the north, south and east.
Ukraine's President Volodymyr Zelenkskyy declared martial law as Russia attacked the country's military infrastructure.
Air-raid sirens went off in the capital Kyiv and explosions were head across Ukraine. Thousands of Kyiv residents fled the city.
World leaders condemned the invasion. German Chancellor Olaf Scholz said "Putin's war" was without justification.