Two planes -- a Gulfstream G650 and a Boeing 787 -- have been grounded after federal prosecutors said their owner, Russian oligarch Roman Abramovich, violated U.S. sanctions by flying the aircraft to Moscow in March.

The Boeing has been in the United Arab Emirates since about March 4, 2022, and the Gulfstream has been in Russia since about March 15, 2022.

The sanctions require a license for any U.S.-made aircraft to fly to Russia. The sanctions also prohibit an aircraft that is owned, controlled or under charter or lease by a Russian national from being flown to Russia.

"No licenses were applied for or issued. Nor was any license exception available, including because the Boeing and the Gulfstream were each owned and/or controlled by a Russian national: Roman Abramovich," said the affidavit supporting a seizure warrant.

The Boeing plane is believed to be among the most expensive private aircraft in the world, worth $350 million, the affidavit said.

-ABC News' Aaron Katersky

Russia has likely moved multiple air defense assets, including SA-15 and SA-22 missile systems, to Snake Island in the western Black Sea, the U.K. Ministry of Defense said Monday in an intelligence update.

The move follows the loss of the Russian warship Moskva, the flagship of Russia's Black Sea Fleet.

"It is likely these weapons are intended to provide air defence for Russian naval vessels operating around Snake Island," the ministry added. "Russia's activity on Snake Island contributes to its blockade of the Ukrainian coast and hinders the resumption of maritime trade, including exports of Ukrainian grain."

Russian forces captured Ukraine's Snake Island in the early days of the invasion, memorably when Ukrainian soldiers defending the tiny islet told an attacking Russian warship to "go f--- yourself." Ukrainian troops have failed in their attempts to retake the previously inconsequential territory.

Meanwhile, in eastern Ukraine's contested Donbas region, heavy fighting continues in the war-torn city of Sieverodonetsk, according to the ministry.

"Russian forces continue to push towards Sloviansk as part of their attempted encirclement of Ukrainian force," the ministry said.

And in the Ukrainian capital, Kyiv, Russian air-launched cruise missiles struck rail infrastructure Sunday in the early morning hours, "likely in an attempt [to] disrupt the supply of Western military equipment to frontline Ukrainian units," according to the ministry.

After five weeks of relative calm in Kyiv, Russian rockets hit Ukraine's capital city on Sunday as Russian President Vladimir Putin warned of strikes on "new targets" if the United States goes through with plans to supply Ukraine with longer-range missiles.

Ukrainian Deputy Minister of Defense Hanna Maliar said the war is still in its "hot phase" and "capturing Kyiv is still Russia’s main goal."

An ABC News crew visited Kyiv's Darnytskyy district, where several Russian cruise missiles slammed into a railway repair plant. One building was still on fire when the ABC News crew arrived. Nearby, another missile strike left a creater on a cement path.

It took hours before Ukrainian authorities permitted media access to the site, saying the area needed to be cleared for safety first.

The Russians claimed the attack in Darnystskyy destroyed military vehicles and armaments. Ukrainian officials said the missiles hit a railway repair plant where no tanks were stored.

Speaking on Russian TV on Sunday, Putin issued a warning to the West on supplying the Ukrainians with high-powered rocket systems. He said if the West carried through with it, Russia would hit "new targets they had not attacked before."

President Vladimir Putin warned that Russian forces would strike new targets if the West began supplying Ukraine with longer-range missiles.

"But if they [missiles] are actually delivered, we will draw appropriate conclusions and apply our own weapons, which we have in sufficient quantities to carry out strikes on targets we aren't striking yet," Putin told Rossyia 1 TV Channel in an interview on Sunday.

-ABC News’ Tanya Stukalova and Tomek Rolski

White House national security advisor Jake Sullivan hinted Thursday of more sanctions coming against Russia in the "next week or two" aimed at targeting ways Moscow is evading sanctions already imposed.

“Where our focus will be over the course of the coming days is on evasion,” Sullivan said Thursday at the Economic Club of Washington. “As Russia tries to adjust to the fact that it’s under this massive economic pressure, what steps do they take to try to evade our sanctions and how do we crack down on that? And I think we'll have some announcements in the next week or two that identify targets that are trying to facilitate that evasion both inside Russia and beyond."

When Sullivan was asked whether sanctions will automatically be lifted if a negotiated peace deal between Russia and Ukraine is worked out, he appeared cautious with his words, saying, “a lot of that depends on what the shape and scope” of the agreement is.

“A lot of it depends on what the Ukrainians, in consultation with us and the Europeans come to agree to," Sullivan said. "You know, we're not going to do a deal over the head of the Ukrainians where we give a bunch of sanctions relief to Russia. But if some measure of sanctions relief were built in to some credible diplomatic solution led by the Ukrainians, that's something that we would happily discuss."

But Sullivan said Russian oligarchs shouldn't expect to ever get back their yachts and other assets seized under sanctions that have been imposed, saying the ultimate goal is "not to give them back” once the war is over.

“The president is actively looking at how we can deal with the fact that as we seize these assets, our goal is not to give them back. Our goal is to put them to a better use than that," Sullivan said. "But I'll be careful in what I say today because there's an ongoing kind of policy process around how we end up dealing with that question. But, rest assured, that the goal is not just to sit on them for a while."

-ABC News' Justin Gomez