US president Biden vows decisive response if Russia attacks Ukraine

US President Joe Biden and Russian President Vladimir Putin spoke Saturday, as tensions over Ukraine reached a fever pitch following remarks by Washington that a Russian invasion was imminent.

Biden spoke to Putin from Camp David, the US presidential retreat in Maryland. The call lasted over an hour.

Biden warned that the US and its allies would "respond decisively and impose swift and severe costs'' if Russia invades, according to the White House. He told Putin that invading Ukraine would cause "widespread human suffering.''

The US is ready to engage in diplomacy with Russia about the situation in Ukraine, but is "equally prepared for other scenarios," Biden told Putin.

A US official, speaking on condition of anonymity, told reporters that the conversation was "professional and substantive," but that there was "no fundamental change in the dynamics unfolding now for several weeks."

It is the first time the two leaders have spoken since December.

Following the call, the Kremlin denounced US "peak hysteria" surrounding the Ukraine crisis, but said Putin and Biden had agreed to continue the dialogue.

The Kremlin's top foreign policy adviser, Yury Ushakov, said Putin complained the West has been arming Ukraine and that Kyiv authorities have been "sabotaging" Western-brokered peace agreements to end a yearslong conflict in eastern Ukraine.

Macron, Scholz, Zelenskyy also engaged

The leaders' phone call came as several European countries have urged their citizens to leave Ukraine as soon as possible. On Saturday, the US cleared non-emergency staff from its embassy in Kyiv and maintain a skeletal team in the western city of Lviv.

Russian Foreign Ministry spokeswoman Maria Zakharova also said Russian diplomatic personnel would be leaving the country's Kyiv Embassy, leaving an "optimized" staff in the Ukrainian capital.

Prior to Saturday's call with Biden, Putin also spoke by phone with French President Emmanuel Macron. The two had met in Moscow earlier in the week to try to resolve what is seen as the biggest security crisis between Russia and the West since the Cold War.

But a summary of the call presented by the Kremlin hinted that progress was not made toward diffusing the situation. Macron's office, meanwhile, said "both expressed a desire to continue dialogue."

Putin shot back at claims of an imminent Moscow invasion of Ukraine, calling it a "provocation."

The Elysee Palace said Macron had informed Putin that "sincere dialogue" was not compatible with an escalation of the situation in Ukraine. Macron consulted with Biden, German Chancellor Olaf Scholz and Ukrainian President Volodymyr Zelenskyy later on Saturday. 

All diplomatic hands on deck

US Secretary of State Antony Blinken spoke with his Russian counterpart, Sergey Lavrov, on Saturday, as did US Defense Secretary Lloyd Austin and Russian Defense Minister Sergei Shoigu, according to the US State and Defense departments.

Blinken reiterated that diplomatic channels of communication would remain open, but, to avoid conflict in Ukraine, Moscow would need to tangibly de-escalate its forces on Ukraine's borders, the State Department said.

Later on Saturday, Blinken said the risk of invasion was currently high enough to justify the decision to reduce diplomatic staff in Ukraine.

"We ordered the departure of most of the Americans still at the US Embassy in Kyiv. The risk of Russian military action is high enough and the threat is imminent enough that this is the prudent thing to do," Blinken told reporters in the US state of Hawaii.

The US has also announced it would remove 150 US military trainers currently located in Ukraine.

What is the possibility of armed conflict?

The confrontation between East and West comes as Russia has amassed over 100,000 troops on Ukraine's borders. Currently, Russia is conducting naval drills in the Black Sea and military exercises in Belarus along Ukraine's northern border. The Kremlin has said it has no intention of invading Ukraine.

Adding to the escalating atmosphere on Saturday was Russia's claim that it had chased off a US submarine that crossed into its territorial waters in the northern Pacific Ocean. The US denied Russia's claim, saying "there is no truth" to the accusations.

The soaring tensions over Ukraine and the possibility of an armed conflict on the European continent are seen as the greatest geopolitical test for NATO and Europe since the Cold War.

Zelenskyy warns panic aids enemies

Despite the increasing signs the scales may be tipping toward an escalation of hostilities, the Ukrainian president told journalists Saturday he was surprised by the latest round of warnings and departures from Western governments.

Zelenskyy said anyone with any information about an invasion should inform the Ukrainian government. He said panic now would only serve Ukraine's enemies.

"We understand all the risks," he said. "We understand that the risks are there."

Australia urges China to 'speak up' on crisis

Australian Prime Minister Scott Morrison has decried Russia's actions as "autocratic and unilateral," but also took aim at China for not speaking up on the crisis.

Morrison, whose government has its own strained ties with China, criticized Beijing for "remaining chillingly silent on Russian troops amassing on the Ukrainian border."

"The coalition of autocracies that we are seeing, seeking to bully other countries, is not something that Australia ever takes a light position on," the prime minister added.

Australia announced Sunday that it is temporarily suspending embassy operations in Kyiv and moved to clear out its diplomatic staff from the country. Canberra also joined dozens of other nations in calling on its citizens to leave, amid fears of a potential Russian attack.