KYIV. Russian military vehicles pushed into Ukraine's second-largest city on Sunday and explosions rocked oil and gas installations on a fourth day of fighting in the biggest assault on a European state since World War Two.

Russian soldiers and armoured vehicles were seen in different parts of the northeastern city of Kharkiv and firing could be heard, a witness said. A burning tank was visible in a video posted by the government.

Russian troops blew up a natural gas pipeline in Kharkiv before daybreak, a Ukrainian state agency said, sending a burning cloud up into the darkness.

"The Russian enemy's light vehicles have broken into Kharkiv, including the city centre," regional Governor Oleh Sinegubov said. "Ukraine's armed forces are destroying the enemy. We ask civilians not to go out."

Ukraine's Western allies ratcheted up their response to Russia's land, sea and air invasion late on Saturday with sanctions to banish major Russian banks from the main global payments system and other measures aimed at limiting Moscow's use of a $630 million war chest of central bank reserves.

Finland and Sweden became the latest European countries to close their airspace to Russian flights, and EU could follow suit with a coordinated European-wide ban, an official said.

Ukrainian forces were holding off Russian troops advancing on the capital Kyiv, President Volodymyr Zelenskiy said. But shelling hit civilian infrastructure and targets including ambulances, he said.

A United Nations agency reported 64 civilian deaths and Ukraine claimed to have killed more than 4,000 Russian soldiers. Reuters was not able to verify the numbers.

More than 368,000 refugees, mainly women and children, have poured into neighbouring countries, clogging railways, roads and borders since Russian President Vladimir Putin, 69, unleashed what he called a special military operation on Thursday.

Ignoring weeks of frantic diplomacy and sanctions threats by Western nations seeking to avoid war, Putin has justified the invasion saying "neo-Nazis" rule Ukraine and threaten Russia's security - a charge Kyiv and Western governments say is baseless propaganda.

The Kremlin sent a diplomatic delegation to neighbouring Belarus offering talks, but Ukraine rejected the offer, saying Belarus had been complicit in the invasion. Ukraine was happy to hold talks elsewhere, Zelenskiy said. Read full story

Russian missiles found their mark overnight, including a strike that set an oil terminal ablaze in Vasylkiv, southwest of Kyiv, the town's mayor said. Blasts sent huge flames and billowing black smoke into the night sky, online posts showed.

"The enemy wants to destroy everything," said the mayor, Natalia Balasinovich.

Ukraine's gas pipeline operator said the transit of Russian gas via Ukraine, vital for Europe's energy needs, was going on as normal. Kremlin-controlled energy giant Gazprom GAZP.M also said gas exports via Ukraine continued normally. Read full story

Russian-backed separatists in the eastern province of Luhansk said a Ukrainian missile had blown up an oil terminal in the town of Rovenky.

Reuters witnesses in Kyiv reported occasional blasts and gunfire through the night, then three blasts after air raid sirens went off shortly before 9 a.m. (0600 GMT).

Ukrainian leaders were defiant.

"We have withstood and are successfully repelling enemy attacks. The fighting goes on," Zelenskiy said in a video message from the streets of Kyiv posted on his social media.

A U.S. defence official on Saturday said Ukraine's forces were putting up "viable" resistance to Russia's air, land and sea advance.

$630 billion war chest

The United States and its allies have authorized more weapons transfers to help Ukraine fight and imposed a range of sanctions on Russia in response to the assault, which threatens to upend Europe's post-Cold War order.

On Saturday, they moved to block certain Russian banks' access to the SWIFT international payment system, making it harder for Russia to trade and for its companies to do business.

They also said they would impose restrictions on Russia's central bank to limit its ability to support the rouble and finance Putin's war effort.

"We will hold Russia to account and collectively ensure that this war is a strategic failure for Putin," the leaders of the European Commission, France, Germany, Italy, Britain, Canada and the United States wrote.

They did not name the banks that would be expelled. An EU diplomat said some 70% of the Russian banking market would be affected. Read full story

Sanctions on Russia's central bank could limit Putin's use of his more than $630 billion in international reserves, widely seen as insulating Russia from some economic harm.

Google barred Russia's state-owned media outlet RT and other channels from receiving money for ads on their websites, apps and YouTube videos, similar to a move by Facebook.

Judo black-belt Putin was suspended as honorary president of the International Judo Federation (IJF) on Sunday, in response to the war. Read full story

'Advancing'

The Kremlin said its troops were advancing again "in all directions" and Putin thanked Russia's special forces, singling out those who are "heroically fulfilling their military duty" in Ukraine.

A Ukrainian presidential adviser said about 3,500 Russian soldiers had been killed or wounded. Western officials have said intelligence showed Russia suffering higher casualties than expected.

Russia has not released casualty figures and Reuters was unable to verify tolls or the precise picture on the ground.

A United Nations relief agency said at least 64 civilians had been killed among 240 civilian casualties.

Ukraine, a democratic nation of 44 million people, won independence from Moscow in 1991 at the fall of the Soviet Union and has pushed to join NATO and the EU, goals Russia opposes.

Putin has said he must eliminate what he calls a serious threat to his country from its smaller neighbour, accusing it of genocide against Russian-speakers in eastern Ukraine - something Kyiv and its Western allies reject as a lie.