MIHAIL KOGALNICEANU AIR BASE, ROMANIA -- Delivering good cheer -- and 5 gallons of ketchup -- Jill Biden thanked U.S. troops deployed to Romania as a check against Russian aggression for their service, as she began a four-day trip to Europe to see first hand the refugee crisis caused by Moscow's invasion of Ukraine.

After flying overnight from Washington, the first lady arrived at Mihail Kogalniceanu Air Base in Romania, near the Black Sea, in time to help serve Friday dinner to U.S. service members stationed there. About 1,600 of the several thousand U.S. troops President Joe Biden deployed to eastern Europe in the leadup to the Russia-Ukraine war were sent to the base, which is about 60 miles (100 kilometres) from Romania's border with Ukraine.

Jill Biden served mac and cheese and baked potatoes for the troops -- and encouraged them not to skip out on the greens -- before greeting small groups of servicemembers at tables in the base dining facility. She delivered bottles of ketchup to the soldiers, responding to a shortage of the condiment on the base, eliciting cheers from the troops.

"I know it's hard on your families," she told one servicemember, referencing her own experience when her son Beau Biden deployed to Iraq.

For weeks, Jill Biden has been transfixed by the news coming out of Ukraine, by the bombings and scenes of "parents weeping over their children's broken bodies in the streets," as she said in a recent speech.

Now Biden is using her second solo overseas trip to get an up-close look at the Ukrainian refugee crisis by visiting Romania and Slovakia, where she will spend Mother's Day meeting with displaced families in a small Slovakian village on the border with Ukraine.

"It's so important to the president and to me that the Ukrainian people know that we stand with them," Jill Biden told reporters travelling with her Thursday night, She said earlier in the week she wants the refugees to know "their resilience inspires me."

NATO allies Romania and Slovakia border Ukraine and have taken in some of the millions of mostly women and children who fled after Russia invaded Ukraine in late February, triggering Europe's largest refugee crisis since World War II.

Biden also will use her four days in Europe to highlight issues she promotes at home, such as support for U.S. service members, education and the welfare of children.

The centrepiece of the first lady's trip comes Sunday -- Mother's Day -- when the mother of three meets with displaced Ukrainians who sought refuge across the border in Slovakia.

Biden's daughter, Ashley Biden, had planned to accompany her mother to Europe, but backed out after learning Thursday that she was a close contact of someone who tested positive for COVID-19, said Michael LaRosa, the first lady's spokesperson. Ashley Biden tested negative, LaRosa said.

"I can only imagine the grief families are feeling," Jill Biden said this week. "I know that we might not share a language, but I hope that I can convey, in ways so much greater than words, that their resilience inspires me, that they are not forgotten, and that all Americans stand with them still."

The first lady also will meet during the trip with humanitarian aid workers, educators, government officials and U.S. embassy personnel, the White House said.

Nearly 6 million Ukrainians, mostly women and children, have fled their country since Russia's invasion, according to the UN refugee agency. Many have resettled in next-door countries, like Romania and Slovakia, or have gone elsewhere in Europe to try to rebuild their lives.

More than 850,000 Ukrainians have entered Romania since the invasion, while nearly 400,000 have crossed into Slovakia, according to government figures from those countries.

Biden has long displayed an interest in the plight of refugees around the world.

In 2011, when her husband was vice president, she travelled to drought-stricken east Africa to visit with Somali famine refugees at the Dadaab camp in Kenya. In 2017, she visited refugees in Chios, Greece, as part of work by the aid organization Save The Children, on whose board she served.

Some refugee advocates said Biden's trip will send the message that the United States takes seriously its humanitarian commitment to the Ukrainian people.

"Every first lady has a far-reaching platform to raise awareness and this trip will be an important tool for mobilizing additional support for those forced to flee their homeland," said Krish O'Mara Vignarajah, president and CEO of Lutheran Immigration and Refugee Service and formerly a policy director to first lady Michelle Obama.

Biden's trip will be the latest to the region by a U.S. government representative following recent visits to Kyiv, Ukraine's capital, by House Speaker Nancy Pelosi, Defense Secretary Lloyd Austin and Secretary of State Antony Blinken to meet with President Volodymyr Zelenskyy.

President Biden visited Ukrainian refugees during a stop in Poland in March. That's the closest he's been to Ukraine. The White House has said there are no current plans for him to visit Kyiv.

After her time with the U.S. service members, Jill Biden was set to spend Saturday in Bucharest, Romania's capital, being briefed on humanitarian efforts, meeting with Romanian first lady Carmen Iohannis and touring a school where Ukrainian refugee students are enrolled before she departs for Slovakia. The first lady is a community college English professor.

On Sunday, she heads to Kosice, Slovakia, to visit a city-operated refugee centre and a public school that also hosts Ukrainian refugee students, where she will spend time with Ukrainian and Slovakian mothers and children as they participate in Mother's Day activities. Afterward, she will travel to the Slovakia-Ukraine border crossing in Vysne Nemecke, Slovakia.

The White House declined to comment on whether she will cross the border and enter Ukraine.

She'll also visit a small Greek Catholic chapel in Vysne Nemecke that serves refugees.

Monday brings a meeting with Slovakian President Zuzana Caputova, the country's first female president, before Biden heads back to Washington.

The first lady has shown her support the Ukrainian people in several ways. She wore a sunflower -- Ukraine's national flower -- on her mask and a dress sleeve and traveled to a Tennessee hospital to visit with Ukrainian children flown there for cancer treatment.

She had Ukraine's ambassador to the U.S., Oksana Markarova, sit with her during President Biden's State of the Union address in March, and she went to the Army's Fort Campbell in Kentucky to visit with the families of U.S soldiers who were deployed to Europe to assist with the Ukraine crisis.

The trip is the first lady's second overseas by herself. She flew to Tokyo last year to represent the United States at the opening of the Olympic Games.