A "small number" of soldiers from the United Kingdom may have "disobeyed orders" by joining Ukraine's fight against invading Russian forces, according to a spokesperson for the British Army.

"We are aware of a small number of individual soldiers who have disobeyed orders and gone absent without leave, and may have travelled to Ukraine in a personal capacity," the British Army spokesperson told ABC News in a statement Wednesday night. "We are actively and strongly encouraging them to return to the U.K."

Personal information on the individuals is not being released for privacy reasons, according to the spokesperson.

The U.K. is advising against all travel to Ukraine and warned that going to fight or assist others engaged in the conflict may be against the law or could lead to prosecution. The U.K., along with its allies, is providing a range of support to Ukraine, including enhancing the country's defense capability. But that support is fundamentally defensive in nature and neither NATO nor Ukraine pose any aggressive threat to Russia, according to a spokesperson for the U.K. Ministry of Defense.

"All Service Personnel are prohibited from travelling to Ukraine until further notice," the U.K. defense ministry spokesperson told ABC News in a statement. "This applies whether the Service Person is on leave or not. Personnel travelling to Ukraine will face disciplinary and administrative consequences."

The spokesperson noted that the U.K. has incredibly limited consular support in Ukraine and is unlikely to be able to offer assistance to any citizens there. There are many ways people can support Ukraine, including through charitable donations, according to the spokesperson, who acknowledged the strong desire to want to help defend freedom and democracy in Europe.

-ABC News' Guy Davies

At least three people, including a child, were killed by a Russian airstrike on a children's hospital and maternity ward in the southeastern port city of Mariupol on Wednesday, according to Ukrainian President Volodymyr Zelenskyy.

Another 17 people, including children, women and health workers, were wounded, Zelenskyy said.

The Ukrainian leader accused Russian state media of lying about the lack of patients inside the hospital at the time of the attack.

"War crimes are impossible without propaganda that covers them," Zelenskyy said in a televised address Thursday. "You will bear responsibility as well as those who give orders to throw bombs on peaceful people."

According to Zelenskyy, 60,000 people managed to evacuate from some areas of Ukraine on Wednesday but Russian forces continue to block Mariupol as well as the small city of Volnovakha.

"Russian troops have already caused a humanitarian catastrophe in Ukraine. But it's just a part of their plan," he said.

Zelenskyy vowed to "rebuild everything" that was destroyed once the war has ended.

"I promise it personally," he added.

European Union leaders are expected to meet in the coming days to discuss Ukraine's application to join the bloc.

"We are waiting for the concrete signal, we are waiting for powerful support because this will be fair, this will be real, this will be human, this will be European," Zelenskyy said. "We know for sure who takes which position in the EU. We know who stands for us and who thinks that the EU is just an accounting department that counts profits, spending and no more."

-ABC News' Yulia Drozd

U.S. Vice President Kamala Harris met with Polish Prime Minister Mateusz Morawiecki and President President Andrzej Duda in Warsaw on Thursday morning, reaffirming the United States' commitment to Poland and other NATO allies.

During a joint press conference with Morawiecki following their bilateral meeting, Harris thanked the Polish people for inviting "with such courage and generosity the refugees who have fled Ukraine."

"As we have said from the beginning, if Russia were to take aggressive action, there would be consequences," Harris added. "And those consequences I believe have been evident but a result of our work together that we have been doing together as a unified force."

Later Thursday, during another joint press conference, reporters asked Harris and Duda about the U.S. rejecting Poland's offer to hand over all its MiG-29 fighter jets to an American air base in Germany to boost Ukraine's fight against Russia. Harris largely dodged the questions on whether the U.S. has an alternative plan for delivering the better air power that Ukraine has request. She pointed to the $13 billion in funding Congress is in the process of passing to give to Ukraine for humanitarian and security needs, in addition to the ongoing support the U.S. has been delivering.

"I can tell you that the issue facing the Ukrainian people and our allies in Eastern flank is something that occupies one of our highest priorities in terms of paying attention to the needs, understanding it is a dynamic situation, and requires us to be nimble and to be swift," she said.

While Duda acknowledged that the situation was an "extremely complicated" one, he argued his country was trying to be a "responsible" and "reliable member of NATO" by addressing the requests made to Poland while working with their partner nations.

"We decided to put those jets at the disposal of NATO, not expecting anything in return," Duda said, "because we stressed very clearly that as a gap filler for the donated equipment, we were able to buy something that we would need as a replacement and we ourselves were ready to provide our equipment free of charge."

-ABC News' Chad Murray and Molly Nagle

More than 2.31 million people have been forced to flee Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the United Nations refugee agency.

The tally from the U.N. High Commissioner for Refugees (UNHCR) amounts to just over 5% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in 15 days.

More than half of the refugees are in neighboring Poland, UNHCR figures show.

-ABC News' Zoe Magee