Russian billionaire Roman Abramovich is taking part in Russian-Ukrainian negotiations at the approval of both parties, although he is not an official member of the Russian delegation, according to Kremlin spokesman Dmitry Peskov.

"Abramovich has been engaged in ensuring certain contacts between the Russian and the Ukrainian side," Peskov said during a daily call with reporters Tuesday. "He is not an official delegation member."

"Nevertheless, he is also present on our side in Istanbul," Peskov added, referring to the peace talks currently underway in Turkey's capital.

Abramovich's participation in the negotiations has been endorsed by both sides, according to Peskov.

Peskov said it will be clear "today or tomorrow whether [the talks] hold some promise or not."

Ukrainian soldiers were seen patrolling the streets of Irpin on Tuesday, following claims that the besieged Kyiv suburb has been "liberated" from Russian forces.

Video released by the Ukrainian military and verified by ABC News shows their troops driving through Irpin, a northwestern suburb of Kyiv where some of the heaviest fighting near the Ukrainian capital has taken place.

Irpin Mayor Oleksandr Markushyn announced in a video posted to Telegram on Monday that "Irpin has been liberated."

Meanwhile, the Ukrainian government released video on Tuesday showing elderly people who had been evacuated from Irpin arriving in Kyiv, where they reunited with their families.

The footage purportedly shows 86-year-old Irpin resident Olga Molchanova being embraced by her daughter and son. Molchanova's adult children had evacuated Irpin earlier via a humanitarian corridor, but their parents refused to leave at the time, according to Ukraine's Center for Strategic Communication and Information Security (Stratcom).

"Recaptured by Ukrainian army, Irpin remains too dangerous for civilians to stay," Stratcom said in a post on Twitter alongside the video.

-ABC News' Monica Camacho and Fergal Gallagher

Russian oligarch Roman Abramovich is taking part in Tuesday's in-person negotiations between Russian and Ukrainian delegations in Turkey's capital, a Turkish presidential source told ABC News.

Abramovich is the owner of the English professional football club Chelsea. The U.K. government has included him among the wealthy Russians targeted in recent sanctions.

The Wall Street Journal reported Monday, citing unnamed sources familiar with the matter, that Abramovich and two Ukrainian negotiators suffered symptoms of suspected poisoning after peace talks in Kyiv in early March.

A Turkish presidential source told ABC News that Abramovich suspected he was poisoned after experiencing eye problems.

Turkish officials confirmed there is a discussion regarding the food protocols during Tuesday's negotiations in Istanbul.

Ukraine is warning its delegation against eating or drinking while attending in-person peace talks with Russian negotiators, amid fears of potential poisoning.

"I advise to everyone who's going to any negotiations with Russia, not to eat or drink anything, and also not to touch any surfaces," Ukrainian Foreign Minister Dmytro Kuleba said in a statement Tuesday.

The warning came a day after The Wall Street Journal reported, , citing unnamed sources, that Russian oligarch Roman Abramovich and two Ukrainian negotiators had suffered symptoms of suspected poisoning following peace talks in Kyiv in early March.

-ABC News' Julia Drozd

A senior administration official told ABC News Friday that the U.S. is getting reports the Russians are starting to bring in biochemical weapon suits into Ukraine.

The reports comes as the Russians accuse Ukraine and the U.S. of developing biochemical weapons, which the U.S. has denied.

"We believe it is an ominous sign they are possibly doing it for cover," the official said.

According to the official, as much as one-fifth of the Russian force is currently "inoperable," meaning they are either dead, wounded or do not have the support or equipment needed to continue.

But even though the Russians are bogged down they realize that everything depends on taking Kyiv and continue to put all effort into that, according to the official.

There are also concerns about boxing Putin in as well as his consistent and casual mention of nuclear weapons. According to the official, that is one of the reasons he has not been publicly labeled as a war criminal because "we just don't know what he will do."

-ABC News' Martha Raddatz