RZESZOW, POLAND (AFP) - US Secretary of State Antony Blinken on Saturday (March 5) visited a crossing on Poland's border with Ukraine where hundreds of thousands of people have been fleeing from advancing Russian forces.

Blinken also visited a cavernous Polish holding centre with around 3,000 refugees and said Washington was preparing to set aside another US$2.75 billion (S$3.7 billion) for the humanitarian crisis.

"The people of Poland know how important it is to defend freedom," he said after talks with Polish Foreign Minister Zbigniew Rau earlier on Saturday in the nearby city of Rzeszow in southeast Poland.

"Poland is doing vital work in response to this crisis."

He said the White House was seeking US$2.75 billion to provide support for those fleeing Ukraine, and countries that accept them, after Russia began its invasion on Feb 24.

Polish border guards on Saturday said around 827,600 people have fled from Ukraine into Poland, making it by far the highest flow of refugees.

Ukraine's other neighbours to the west, Hungary, Moldova, Romania and Slovakia, have also taken in refugees.

Speaking next to Blinken, Rau said Poland would remain open to refugees.

"Russia's aggression in Ukraine caused a humanitarian crisis of an unimaginable scale," said Rau.

"Our priority is organising effective aid to hundreds of thousands, and soon to be millions of refugees."

Rau also pledged not to discriminate between refugees of different nationalities, after reports circulated in Washington that Africans and others fleeing from Ukraine were being impeded at the border with Poland.

He accused Russian forces of committing "war crimes" by shelling in residential areas.

After meeting with Blinken, Polish Prime Minister Mateusz Morawiecki called for tougher sanctions against Russia saying they should be "hitting Vladimir Putin's war machine".

Morawiecki called for all Russian banks to be excluded from the SWIFT payment system and said asset freezes "should be as extensive as possible".

Blinken is visiting key allies in Europe to demonstrate Washington's support for their security and shore up Western unity against Moscow.

He will travel next to Moldova, which has also experienced an inflow of Ukrainians, and the three Baltic states, particularly concerned about Russia.

Blinken spent Friday in Brussels in meetings with counterparts at Nato and the European Union to discuss putting more pressure on Russia and additional support for refugees.

Follow The Straits Times' live coverage on the Ukraine crisis here.