KOSICE/VYSNE NEMECKE, Slovakia (Reuters) -U.S. first lady Jill Biden met Ukrainian refugees in eastern Slovakia on Sunday, the last full day of her tour of Romania and Slovakia to visit U.S. servicemen deployed there and women and children who fled Russia's invasion of Ukraine.

Biden spoke to families, volunteers and local authority workers at a refugee centre in the eastern Slovak city of Kosice, one of the main transit points for over 400,000 Ukrainian refugees who have crossed the border to Slovakia.

The United Nations says 5.8 million people in total have fled Ukraine since Russia started what it describes as a "special military operation" in Ukraine on Feb. 24.

"When the war started we understood that nowhere in Ukraine is safe," Viktoria Kutocha, a teacher who fled the western city of Uzhhorod with her 7-year old daughter, told Biden.

Biden, the wife of U.S. President Joe Biden, asked Kutocha how she explained the war to children.

"It's very difficult to explain. I only said there is a war and I cannot explain because I do not know myself," Kutocha said.

"It's senseless," Biden responded, before embracing the mother and her child.

Biden, who teaches English and writing at a community college in Virginia, also visited a local school attended by refugee children.

"The hearts of the American people are with the mothers of Ukraine," she said, wishing the women at the school a happy Mother's Day.

One woman there said she felt children needed to have the war explained to them.

"They need to understand why we are here and why we receive help - why are we separated from our husbands. They need to understand what is going on," she said.

Later on Sunday, Biden and Slovak Prime Minister Eduard Heger visited Vysne Nemecke, the main road crossing on Slovakia's 98 km (61 mile) border with Ukraine.

She expressed gratitude to officials and non-government organisations, and joined a brief prayer for refugees in an improvised chapel in one of many tents set up by aid groups.

Biden will leave the region on Monday after a four-day tour.

Most refugees who crossed into Slovakia have travelled onward to other countries, but over 74,000 have received temporary protection status in the country of 5.5 million.