Air raid sirens wailed over Kyiv amid unconfirmed reports that a Russian plane had been shot down and crashed into a building a day after Russian President Vladimir Putin launched an invasion that has shocked the world.

A senior Ukrainian official said Russian forces would enter areas just outside the capital, Kyiv, later on Friday and that Ukrainian troops were defending positions on four fronts despite being outnumbered.

An estimated 100,000 people fled as explosions and gunfire rocked major cities. Dozens have been reported killed. Russian troops seized the Chernobyl former nuclear power plant north of Kyiv as they advanced on the city from Belarus.

US and Ukrainian officials say Russia aims to capture Kyiv and topple the government, which Putin regards as a puppet of the United States.

Zelenskiy said he understood Russian troops were coming for him but vowed to stay in Kyiv.

"(The) enemy has marked me down as the number one target," Zelenskiy said in a video message. "My family is the number two target. They want to destroy Ukraine politically by destroying the head of state."

"I will stay in the capital. My family is also in Ukraine."

Russia launched its invasion by land, air and sea on Thursday following a declaration of war by Putin, in the biggest attack on a European state since World War II.

Putin says Russia is carrying out "a special military operation" to stop the Ukrainian government from committing genocide against its own people - an accusation the West calls baseless. He also says Ukraine is an illegitimate state whose lands historically belong to Russia.

Ukrainian forces downed an enemy aircraft over Kyiv early on Friday, which then crashed into a residential building and set it ablaze, said Anton Herashchenko, an adviser to the interior minister.

It was unclear whether the aircraft was manned or whether it could be a missile. Kyiv municipal authorities said at least eight people were injured when the object crashed into an apartment block.

"Horrific Russian rocket strikes on Kyiv," Foreign Minister Dmytro Kuleba wrote on Twitter. "Last time our capital experienced anything like this was in 1941 when it was attacked by Nazi Germany."

A border post in the southeastern Zaporizhzhya region had been hit by missiles, causing deaths and injuries among border guards, and air raid sirens sounded over the city of Lviv in the west of the country.

A democratic nation of 44 million people, Ukraine voted for independence at the fall of the Soviet Union and has recently stepped up efforts to join the NATO military alliance and the European Union, aspirations that infuriate Moscow.

The United States, Britain, Japan, Canada, Australia and the EU unveiled more sanctions on Moscow on top of penalties earlier this week, including a move by Germany to halt an $US11 billion gas pipeline from Russia.

EU foreign policy chief Josep Borrell described the bloc's measures as "the harshest package of sanctions we have ever implemented".

China came under pressure over its refusal to call Russia's assault an invasion.

US President Joe Biden, speaking to reporters at the White House, said: "Any nation that countenances Russia's naked aggression against Ukraine will be stained by association."

The UN Security Council will vote on Friday on a draft resolution that would condemn Russia's invasion and require Moscow's immediate withdrawal. However, Moscow can veto the measure, and it was unclear how China would vote.

Russia is one of the world's biggest energy producers, and both it and Ukraine are among the top exporters of grain. War and sanctions will disrupt economies around the world.

Oil prices soared as much as $US2 per barrel on Friday as markets brace for the impact of trade sanctions on major crude exporter Russia.

Australian Associated Press