Poland's border guards on Sunday said nearly 200,000 people had crossed into the European Union member from Ukraine in the days since Russia invaded the ex-Soviet country.

The latest figure represented an increase of tens of thousands on what was reported Saturday, showing just how quickly Ukrainian refugees are flowing into their western neighbor.

"Over the last three and a half days, nearly 200,000 people have received assistance on our territory," said Tomasz Praga, the head of the Polish border guard. He told reporters that on Saturday alone more than 77,000 people had crossed into Poland from Ukraine.

Poland, which was already home to an estimated 1.5 million Ukrainians before Russia's invasion Thursday and which has expressed steadfast support for Ukraine, has so far seen the bulk of those fleeing Ukraine cross into its territory.

But thousands of others have fled to fellow Ukraine neighbors Hungary, Moldova, Slovakia and Romania.

While tens of thousands of refugees are leaving Ukraine amid Russia's attack on the country, some Ukrainian men and women are returning home from across Europe to help defend their homeland.

At the checkpoint in Medyka, in southeastern Poland, many were standing in a line early Sunday to cross into Ukraine.

"We have to defend our homeland. Who else if not us," said a moustachioed man in front of a group of some 20 Ukrainian truck drivers walking to the checkpoint to enter Ukraine. They came from across Europe to return to Ukraine. They spoke to the Associated Press (AP) in Ukrainian and in Russian.

Another man in the group said: "The Russians should be afraid. We are not afraid."

Members of the group declined to give their names, or only gave their first names, citing their security and that of their families.

A woman in her 30s who gave her first name, Lesa, spoke to the AP just before entering the checkpoint building. "I am afraid, but I am a mother and want to be with my children. What can you do? It's scary but I have to."

Another young woman said she, too, was returning to take care of her children, so that Ukrainian men can defend the country.

"We have to, we Ukrainians have to take our children away... to allow our boys to fight," she said.

At least 150,000 people have fled Ukraine into Poland and other neighboring countries in the wake of the Russian invasion, the United Nations refugee agency, the UNHCR, said Saturday. It did not give the figures for those going to Ukraine.