Polish Deputy Interior Minister Pawel Szefernaker on Saturday said 100,000 people have crossed the border into Poland from Ukraine since Russia's invasion this week.

"From the onset of warfare in Ukraine through today, along the entire border with Ukraine, 100,000 people have crossed the border from Ukraine into Poland," Szefernaker told reporters in the border village of Medyka, southeastern Poland. 

The head of the Polish border guard, Tomasz Praga, added at the press conference that on Friday alone nearly 50,000 people had crossed into Poland from Ukraine. 

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

"Latest update is that almost 116,000 have fled to neighbouring countries since 24 Feb -- mainly Poland, Hungary, Moldova, Slovakia and Romania," the UN refugee agency UNHCR tweeted on Saturday. 

"Numbers are rising," it added. 

Szefernaker said 90 percent of the refugees have concrete places to go in Poland, such as the homes of friends or family, but that the remainder are seeking help at nine reception centres set up along the border.

The centres offer meals and medical care, a place to rest as well as any necessary information. Those refugees with nowhere to go are then taken by bus to various accommodation options prepared in advance by Poland.

Separately, its Infrastructure Minister Andrzej Adamczyk announced Saturday that "for the next four weeks, Ukrainian citizens will be able to ride trains operated by (Polish rail service) PKP Intercity free of charge."

The EU member of 38 million people has also readied an ambulance train in the border city of Przemysl. 

Polish government official Michal Dworczyk said that if and when necessary the train will travel to the Ukrainian border city of Mostyska, pick up the wounded and transport them to Warsaw for hospital treatment. 

"I hope that we won't have to use these railroad cars," he told reporters. 

He added that Poland is doing everything it can to help its neighbour under attack. 

"Over the last few days we've seen an incredible burst of solidarity among Poles," he said. 

"I have yet to come across someone who didn't want to somehow help."