US Secretary of State Antony Blinken arrived in Poland on Saturday for talks with officials as hundreds of thousands of Ukrainian refugees pour into the country to escape the conflict.

The most senior US official to visit Poland since the war began, Blinken will hold talks with Polish Prime Minister Mateusz Morawiecki and Foreign Minister Zbigniew Rau in Rzeszow, near the border with Ukraine.

More than 780,000 people have fled Ukraine into Poland since the invasion began on February 24.

Including other neighbouring countries, more than a million have left Ukraine.

Blinken is visiting key allies in Europe that are feeling the pressure from the war, to demonstrate Washington’s support for their security and shore up Western unity against Moscow.

Following his stop in Poland he will travel to Moldova, which has also experienced an inflow of Ukrainians, and the three Baltic states which are particularly concerned by Russian actions.

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

"The Kremlin's attacks are inflicting an ever increasing toll on civilians there. Hundreds if not thousands of Ukrainians have been killed, many more wounded," he said in Brussels Thursday.

"More than a million refugees have fled Ukraine to neighboring countries," he said, pledging more support from Washington.