The U.K. has detained a $50 million, Russian-owned superyacht in London, where it was docked in the Canary Wharf financial district. 

The yacht, named Phi, was detained under the government's Russian sanctions, with Britain's National Crime Agency saying it was the first detention of a superyacht in U.K. waters. 

It said officers had served a detention notice on the superyacht Tuesday morning after confirming the identity of its owner.

"Owned by a Russian businessman, Phi is the third biggest yacht built by prestigious shipbuilders Royal Huisman and includes what the builders call an 'infinite wine cellar' and patented fresh-water swimming pool," the NCA said in a press release. 

It said the ownership of the yacht had been "deliberately well hidden," with the company the ship was registered to being based in St. Kitts and Nevis and carrying Maltese flags "to hide its origins."

Ukrainian negotiators involved in peace talks with Russia in Istanbul on Tuesday said they want to see an international agreement under which other nations would act as guarantors of Ukraine's security. 

David Arahamiya, a member of Ukraine's delegation, said the Ukrainian side wanted international security guarantees that would see guarantor countries act in a similar way to NATO's Article 5, which holds that an attack against one member country should be seen as an attack against all members.

He said guarantor countries could include United Nations Security Council member states, including the United States, United Kingdom, China, Russia and France, as well as other countries such as Turkey, Canada, Germany, Italy, Poland and Israel. If other countries wanted to join, they would have the opportunity, he said. 

Arahamiya said there would also be a clause in the agreement stating that all guarantor countries will not only refrain from denying Ukraine's accession to the European Union, but would also assist in the effort. 

He said the agreement would first have to be approved in a referendum by Ukrainians and then ratified by the Parliament of Ukraine and approved by guarantor countries. 

A Ukrainian soldier stands near the wreck of a Russian tank on the front line in the Kyiv region Monday.

Russia will "drastically" reduce military activity near the Ukrainian capital of Kyiv and in the nearby city of Chernihiv, Russian Deputy Defense Minister Alexander Fomin said Tuesday.

Fomin announced the decision in an on-camera statement from Istanbul, where peace talks between Ukrainian and  Russian delegations were underway. His comments were broadcast on Russia-24.

He said the the general staff of the Russian armed forces would report on the decision in more detail upon the return of Russia's delegation to Moscow. 

At least 7 people have been killed after a Russian strike hit the regional government building in the southern Ukrainian city of Mykolaiv, the State Emergency Service of Ukraine has said. 

Earlier, it had said at least three people had died. NBC News was not able to confirm a death toll. 

The state emergency service said as many as 22 others were injured in the attack. 

The regional governor, ​​Vitaliy Kim, had said on Telegram on Tuesday that his personal office was also destroyed. He said most people appeared to have survived the attack. 

Wounded women sit on their beds in a hospital in Zaporizhzhya, Ukraine, on Tuesday.

Security guarantees for Ukraine will be a key issue in ongoing talks between Ukrainian and Russian delegations in Turkey on Tuesday, Zelenskyy adviser Mykhailo Podoliak has said.

Speaking on Ukrainian television during a break in the talks in Istanbul, he said security guarantees would be a primary focus.

Podoliak said efforts would also be made to negotiate a cease-fire to address the growing humanitarian issues in besieged cities. 

"Unconditional security guarantees for Ukraine, ceasefire, effective decisions on humanitarian corridors and humanitarian convoys, observance by the parties of the rules and customs of war," he tweeted. "Difficult negotiations for peace in our country," he said.

The International Committee of the Red Cross called on Ukraine and Russia on Tuesday to reach a clear agreement for the safe evacuation of civilians from the besieged southern Ukrainian city of Mariupol and other places as vital supplies run out.

Robert Mardini, ICRC director-general, told Reuters that the neutral aid agency would not participate in any forced evacuations of civilians from Ukraine and it had no first-hand information that this is happening. 

"Our concern is that the very intensity of the fighting is putting civilians in harm's way, the fact that in places like Mariupol civilians are not able to leave in safe conditions, there were no concrete agreements by parties to the conflict for safe evacuation of civilians, nor has there been a green light to get humanitarian aid in," Mardini said.

Ukraine and Russia must allow the ICRC to visit captured prisoners of war, in line with the Geneva Conventions, and return the remains of people killed in the conflict, he said in an interview at ICRC headquarters in Geneva.

Russian oligarch Roman Abramovich listens during peace talks between Russian and Ukrainian delegations in Istanbul on Tuesday.