Russian forces have attacked oil and gas facilities in Ukraine, sparking huge explosions, officials said on Sunday, as Western allies prepared new sanctions, including banishing key Russia banks from the main global payments system.

Ukrainian forces were holding off Russian troops advancing on the capital, Kyiv, President Volodymyr Zelenskiy said as the biggest assault on a European state since World War Two entered its fourth day.

But Zelenskiy said the night had been brutal, with shelling of civilian infrastructure and attacks on everything, including ambulances.

Russian President Vladimir Putin launched what he called a special military operation on Thursday, ignoring weeks of Western warnings and saying the "neo-Nazis" ruling Ukraine threatened Russia's security - a charge Kyiv and Western governments say is baseless propaganda.

Offering a glimmer of hope for negotiations, the Kremlin said a delegation had arrived in neighbouring Belarus for talks and was waiting for the Ukrainians. But Zelenskiy rejected talks in Belarus, saying it was complicit in the invasion, but he left the door open for negotiations elsewhere.

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.

There was heavy fighting for Ukraine's second city, Kharkiv, in the northeast, where Russian troops blew up a natural gas pipeline, a Ukrainian state agency said.

That blast sent a mushroom cloud up into the darkness, though Ukraine's gas pipeline operator said the transit of Russian gas to Europe via Ukraine was going on as normal.

Russian troops later entered Kharkiv, interior ministry adviser Anton Herashchenko said on Telegram. Videos posted by him and a state agency showed several military vehicles moving on a street and, separately, a burning tank.

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 said Ukraine's forces were putting up "very determined resistance" to Russia's air, land and sea advance, which has sent hundreds of thousands of Ukrainians fleeing westwards, clogging major highways and railway lines.

Bad for business

The United States and its allies have imposed a range of sanctions on Russia in response to an assault that 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. 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 are resolved to continue imposing costs on Russia that will further isolate Russia from the international financial system and our economies," said a statement from the United States, France, Germany, Canada, Italy, Britain and the European Commission.

After initially shying away from such a move largely because of concern about the impact on their economies, the allies said they committed to "ensuring that selected Russian banks are removed from the SWIFT messaging system."

They did not name the banks that would be expelled, but an EU diplomat said some 70% of the Russian banking market would be affected. read more

The decision - which the French finance minister had called a "financial nuclear weapon" because of the damage it would inflict on the Russian economy - deals a blow to Russia's trade and makes it harder for its companies to do business.

SWIFT, a secure messaging network that facilitates rapid cross-border payments, said it was preparing to implement the measures.

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 move Facebook made.

Advancing

Earlier, the Kremlin said its troops were advancing again "in all directions" after Putin ordered a pause on Friday. Ukraine's government said there had been no pause.

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 it was impossible to verify tolls or the precise picture on the ground.

A United Nations relief agency said as of Saturday evening at least 64 civilians had been killed among 240 civilian casualties, more than 160,000 people had been internally displaced and more than 116,000 had fled to neighbouring countries.

Ukraine, a democratic nation of 44 million people, won independence from Moscow in 1991 after the fall of the Soviet Union and wants 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.