Joe Biden met with Ukraine's top government officials in Warsaw

Biden dropped in on a meeting between meeting between Ukraine's Foreign Minister Dmytro Kuleba and Defense Minister Oleksii Reznikov and Secretary of State Anthony Blinken and Defense Secretary Lloyd Austin.

The United States expressed "unwavering commitment to Ukraine’s sovereignty and territorial integrity," State Department spokesman Ned Price said.

Kuleba told reporters that Ukraine had received additional security pledges from the United States on developing defence co-operation.

"President Biden said what is happening in Ukraine will change the history of the 21st century, and we will work together to ensure that this change is in our favour, in Ukraine's favour, in the favour of the democratic world," Kuleba told Ukrainian national television soon after.

Later in the day Biden will give what his aides have billed as a major speech.

The White House said that Biden "will deliver remarks on the united efforts of the free world to support the people of Ukraine, hold Russia accountable for its brutal war, and defend a future that is rooted in democratic principles".

Biden has held three days of emergency meetings with allies in the G7, Europe and NATO, and visited with U.S. troops in Poland on Friday. He met Polish President Andrzej Duda on Saturday.

Biden is also scheduled to visit a refugee reception centre at Warsaw's national stadium. More than 2 million people have fled the war to Poland, out of the roughly 3.8 million who have left Ukraine all together.

President Vladimir Putin's Feb. 24 invasion of Ukraine, which Russia calls a "special operation", has tested NATO and the West's ability to unite in the face of a war at the borders of NATO countries.

Poland was until the collapse of communist rule in 1989 behind the Iron Curtain for four decades, under Soviet influence and a member of the Moscow-led Warsaw Pact security alliance. It is now the biggest formerly communist member of the European Union and NATO.

The rise of rightwing populism in Poland in recent years has put it in conflict with the European Union and Washington, but the threat of Russia pressing beyond its borders has drawn Poland closer to its Western neighbours.

Biden's election put the nationalist Law and Justice government in an awkward position after it had set great store in its relationship with his predecessor Donald Trump.

But as tensions with Russia rose before it invaded Ukraine, Duda appeared to seek to smooth relations with Washington. In December, he vetoed legislation that critics said aimed to silence a U.S.-owned TVN24 news channel.

Biden and Duda were exected in their meeting to address a dust-up over how to arm Ukraine with warplanes, and other security guarantees.

Washington, seeking to avoid a direct conflict with Russia, earlier this month rejected a surprise offer by Poland to transfer Russian-made MiG-29 fighter jets to a U.S. base in Germany to be used to replenish Ukraine's air force.

Now, Poland wants to accelerate the purchase of U.S.-made Patriot missiles, F35 fighter jets and tanks for its own security, and seek reassurance on NATO commitments to defend its members.