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

Russian forces on Saturday night attacked cities throughout eastern Ukraine, with shelling near Mariupol and Kharkiv, Ukraine's General Staff of the Armed Forces said in a statement on Sunday.

Russian and Ukrainian forces clashed overnight near Izyum, near Kharkiv, in the Donetsk Oblast, as Russian troops attempted to "break through" the local defense, the statement said.

"The East of Ukraine is on fire again," Lesia Vasylenko, a government minister, said. "All news of the morning: troops amassing around Kharkiv, artillery attacks all across towns in Donetsk and Luhansk regions. The same geography as in 2014/15, just wider. Russian aggression continues."

Russian missiles fired during the night hit a military base near Zvonetskoe, west of Izyum, Russia's Armed Forces said in a Sunday morning update. Moscow claimed its missiles also had hit Chuhuev military airfield, near Kharkiv, destroying anti-aircraft missile launchers and systems.

Ukraine's military claimed to have repelled eight Russian attacks, destroying four tanks, since Saturday morning.

The U.K. Ministry of Defense on Saturday warned that Russia's attacks throughout the country carried high risks to civilians.

"Russian forces continue to use IEDs to inflict casualties, lower morale, and restrict Ukrainian freedom of movement," the ministry said. "Russian forces also continue to attack infrastructure targets with a high risk of collateral harm to civilians, including a nitrate acid tank at Rubizhne."

Ukrainian President Volodymyr Zelenskyy praised U.K. Prime Minister Boris Johnson for visiting Kyiv Saturday and reiterated his call to other European leaders for support during his daily address.

Zelenskyy said the British leader's visit demonstrated "there are no obstacles to freedom," and "no obstacles to leadership."

"The leadership of the United Kingdom in providing our country with the necessary assistance, especially in terms of defense, as well as the leadership in sanctions policy will remain forever in history," he said. "Ukraine will always be grateful to Boris and Britain for this."

Zelenskyy said he also spoke with Austrian Chancellor Karl Nehammer on Saturday. Nehammer "supported a clear prospect of Ukraine's accession to the European Union as soon as possible," according to Zelenskyy.

Zelenskyy said it was a moral duty for all democracies to support Ukraine and to increase pressure on Russia.

The president also relished that Ukrainian forces have defended their country for 45 days.

"Could Moscow think that such events would take place in Kyiv on the 45th day after their invasion? No, they couldn't even imagine it," he said. "And we made it a reality."

-ABC News' Jason Volack

After meeting in Kyiv, Ukrainian President Volodymyr Zelenskyy and British Prime Minister Boris Johnson held a joint press conference, telling reporters Russian forces are expected to intensify pressure in Donbas and in the east.

"What Putin has done in places like Bucha and in Irpin his war crimes, have permanently polluted his reputation and the reputation of his of his government," Johnson said.

He added, "It's clear that what he's doing now, he has suffered a reverse, but his retreat is tactical, and he's going to intensify the pressure now in Donbass and in the East."

Zelenskyy praising the U.K.'s sanctions, called for increased pressure on Russia and a boycott of Russian energy.

"The other democratic Western countries should follow the example of the United Kingdom. It's time to impose a complete embargo on Russian energy resources, which they should increase the amount of weapons being supplied," Zelenskyy said.

Johnson warned that the U.K. and its partners are going to increase economic pressure and will move away from the use of Russian hydrocarbons.

"The defense intelligence that we had suggested that the Russians believed that Ukraine could be engulfed in a matter of days and that Kyiv would fall in hours to them to their armies, and how wrong they were," Johnson said.

"And I think that the Ukrainians have shown the courage of a lion, but you, Vladimir, have given the roar of that lion," he added.

Johnson later said, "what this war is certainly producing is a clarity about the vision of a future for Ukraine, where, together with friends and partners, we, the U.K. and others supply the equipment, the technology, the intelligence so that Ukraine will never be invaded again. So that Ukraine is so fortified and so protected that Ukraine can never be bullied again, never be blackmailed again, never be threatened in the same way again."

Ukrainian President Volodymyr Zelenskyy said he is committed to pressing for peace despite Russia's attacks on civilians, according to The Associated Press.

Zelenskyy also called for countries to send Ukraine more weapons ahead of an expected surge in fighting in the east, according to the AP.

"No one wants to negotiate with a person or people who tortured this nation. It's all understandable. And as a man, as a father, I understand this very well," Zelenskyy told The AP. But "we don't want to lose opportunities, if we have them, for a diplomatic solution."

"We have to fight, but fight for life. You can't fight for dust when there is nothing and no people. That's why it is important to stop this war," he said to The AP.

Zelenskyy told the AP he is confident Ukrainians would accept peace despite the horrors they witnessed in the war.