Russia has launched renewed missile attacks on Ukraine , with the president's chief of staff warning that the capital, Kyiv, was also under attack.
"[Russian President Vladimir] Putin is launching a missile attack on Kyiv right now," Andriy Yermak said on Telegram.
Ukraine's air force previously warned that a missile had entered the country's airspace and was headed for Kyiv.
Reuters' witnesses reported hearing blasts in the city.
"Explosions in the city. Air defense forces are working. Stay in shelters!" the Kyiv city administration said on Telegram.
The attacks involved cruise missiles launched from strategic aircraft as well as ballistic missiles, the Ukrainian military said.
Thousands of North Korean troops in action in Russia's Kursk, US says
The majority of a 10,000-strong contingent of North Korean troops deployed to Russia is already taking part in combat operations in the Kursk region bordering Ukraine, a US State Department spokesman said.
But how effective the North Korean soldiers will be "will in large part be dictated by how well the Russians can integrate them into their military," according to the department's deputy spokesman, Vedant Patel.
"Some of the challenges they would need to overcome are interoperability, the language barrier, command and control, and communications," he added.
Ukrainian President Volodymyr Zelenskyy reported a few days ago that some of the North Korean contingent were already taking part in fighting against the Ukrainian army, which has succeeded in capturing some territory in the Russian border region over the past months.
The presence of the North Korean troops in Russia has fueled fears that the conflict in Ukraine will spill over into a wider proxy war.