Urgent efforts are continuing to evacuate the most medically vulnerable populations out of Ukraine.

Dubbed in European media as the "Train of Hope," essentially a rolling hospital with several cars filled with beds and medical equipment, the locomotive carried child cancer patients Thursday after it left an obscure train stop along Poland’s border with Ukraine.

The children, who were visibly exhausted and accompanied by parents clearly in distress, will travel on an eight-hour journey to Gdynia, a city in northern Poland.

The train was initially set up by Poland's Ministry of Health to transport the wounded from fighting in Ukraine to hospitals in Poland. Instead, it has been used to carry hundreds of refugees, along with sick children and pregnant mothers, to Poland.

One medic told ABC News that the maiden trip two weeks ago was intended to be a training mission. But when they arrived to a sea of people, the crew decided to evacuate 600 refugees — the majority of whom were children fighting cancer, the medic said.

-ABC News' Chris Donato and Marcus Moore

Russia's Defense Ministry spokesman on Thursday claimed it did not carry out the attack on a children's hospital in Mariupol, which Ukrainian officials said killed 3 people and injured 17.

Russian officials, including its foreign minister, had earlier said they targeted the hospital, claiming it was being used as a firing position by Ukrainian nationalists.

Russia’s Defense Ministry spokesman claimed that the devastating bombing was staged.

-ABC News' Patrick Reevell

Secretary of State Antony Blinken spoke to his Polish counterpart, Foreign Minister Zbigniew Rau, to discuss joint security and humanitarian efforts in Ukraine, specifically Russia's attacks on civilian population centers, according to the U.S. State Department.

The two met on the Polish-Ukrainian border Saturday. The phone call comes amid disagreements over sending fighter jets to Ukraine after Poland said it is willing to send the jets to a U.S. base in Germany, the Pentagon said the move could be seen by Russia as "escalatory."

"Secretary Blinken and Foreign Minister Rau discussed ongoing diplomatic efforts, including sanctions, to stop Putin’s war of aggression. The Secretary reiterated the United States’ steadfast support for Ukraine’s sovereignty and territorial integrity," the U.S. State Department said in a release.

The State Department added, "The Secretary and Foreign Minister also discussed the continued importance of providing security assistance to Ukraine."

The call comes as Vice President Harris visits Poland, and after Defense Secretary Lloyd Austin called his Polish counterpart on Wednesday.

Blinken also spoke to Ukrainian Foreign Minister Dmytro Kuleba Wednesday, ahead of his talks with Russian Foreign Minister Sergey Lavrov.

-ABC News' Conor Finnegan

Senate Minority Leader Mitch McConnell decried the Biden administration on the Senate floor Thursday over the "bizarre argument" that played out in public after the U.S. refused to accept Poland’s offer to donate MiG-29 aircrafts to Ukraine’s military resistance.

McConnell accused the Biden Administration of "contradicting themselves" and of proving incapable to coordinate assistance efforts with NATO allies.

"The world needs President Biden and his team to be leading, taking the initiative and shaping circumstances, calmly soberly, but with confidence and decisiveness," McConnell said. "They seem to be trapped in a cycle of passivity indecision self-determines."

-ABC News' Trish Turner