Reuters - 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.

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.

But 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.

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.