Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

British Prime Minister Boris Johnson met with Ukrainian President Volodymyr Zelenskyy in Kyiv on Saturday, according to the Ukrainian Embassy in London.

"The Prime Minister of the United Kingdom Boris Johnson is one of the most principled opponents of the Russian invasion, a leader in sanctions pressure on Russia and defence support for Ukraine. Congratulations in Kyiv, my friend!" Zelenskyy said in a post online.

Britain announced it will send 120 armored vehicles and new anti-ship missile systems to support Ukraine, according to the The Associated Press.

"Today I met my friend President @ZelenskyyUa in Kyiv as a show of our unwavering support for the people of Ukraine. We're setting out a new package of financial & military aid which is a testament of our commitment to his country's struggle against Russia’s barbaric campaign," Johnson said.

Ukraine claimed Saturday that its intelligence indicates Russian forces are currently regrouping in eastern Ukraine, then the tankers intend to move toward Kharkiv, Ukraine's Defense Intelligence Chief Kyrylo Budanov said.

Russians are also regrouping in the direction of Izum, in the Kharkiv region, the Ukrainians said.

"They plan to enter Kharkiv in the first place. They will try to conquer the city of Mariupol and only after that they can try to initiate Kiev," Budanov said in an interview on CNN.

To compensate for their significant losses in Ukraine, the Russian army is transferring additional troops to Belgorod (Russia), from where they are relocated to Kharkiv region, the Ukrainians claimed.

Budanov said Ukraine needs serious military support from allies so its soldiers can more effectively resist Russian aggression.

-ABC News' Mike Trew

Ukrainian President Volodymyr Zelenskyy called Friday's deadly missile attack on the Kramatorsk train station in eastern Ukraine "another war crime of Russia."

"All the world's leading powers have already condemned Russia's attack on Kramatorsk," Zelenskyy said during his latest national address Friday. "We expect a firm, global response to this war crime."

The president called for the missile strike, which killed at least 50, to be among the charges in a war crimes tribunal against Russia.

"All the efforts of the world will be aimed to establish every minute: who did what, who gave orders. Where did the rocket come from, who was carrying it, who gave the order and how the strike was coordinated," Zelenskyy said. "Responsibility is inevitable."

During his daily press briefing, Pentagon spokesman John Kirby was clear that the U.S. believes that Russia was behind the rocket attack on a train station in eastern Ukraine earlier Friday.

"We find unconvincing Russian claims that they weren't involved, particularly when the ministry actually announced it and then when they saw reports of civilian casualties they decided to unannounce it,” Kirby said during Friday afternoon's briefing. "So our assessment is that this was a Russian strike and they used a short-range ballistic missile to conduct it."

Kirby called it a part of the trend by Russia of "brutality" and "carelessness" in not avoiding civilian casualties as they carry out this war in Ukraine.

At least 50 people, including five children, were killed in the rocket attack in Kramatorsk, Ukrainian authorities said.

A Kremlin spokesman denied involvement in the attack, saying Russia's Armed Forces do not use the type of missile used in the strike and that no combat tasks were planned in the region.

-ABC News' Luis Martinez