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KYIV, Ukraine — Talks to stop the fighting in Ukraine resumed Friday, as another attempt to rescue civilians from the besieged port city of Mariupol broke down and Russia accused the Ukrainians of launching a cross-border helicopter attack on an oil depot.

The governor of Russia’s Belgorod region said the alleged airstrike by a pair of helicopter gunships caused multiple fires and injured two people. A Kremlin spokesman said the incident on Russia’s territory could undermine the negotiations between Russian and Ukrainian representatives.

“Certainly, this is not something that can be perceived as creating comfortable conditions for the continuation of the talks,” Kremlin spokesman Dmitry Peskov replied when asked if the strike could be viewed as an escalation of the war in Ukraine.

It was not immediately possible to verify the claim that Ukrainian helicopters targeted the oil depot or several nearby businesses in Belgorod also reported hit. Russia has reported shelling from Ukraine before, including an incident last week that killed a military chaplain, but not an incursion of its airspace.

Asked if Ukraine had fired on the depot, Ukrainian Foreign Minister Dmytro Kuleba said in Warsaw that he could “neither confirm nor nor reject the claim that Ukraine was involved in this simply because I do not possess all the military information.”

The latest negotiations, taking place by video link, follow a meeting in Turkey on Tuesday where Ukraine reiterated its willingness to abandon a bid to join NATO and offered proposals to have its neutral military status guaranteed by a range of foreign countries.

The head of the Russian delegation, Vladimir Medinsky, wrote on social media that Moscow’s positions on retaining control of the Crimean Peninsula and expanding the territory in eastern Ukraine held by Russia-backed separatists “are unchanged.”

The International Committee for the Red Cross said complex logistics were still being worked out for the operation to get emergency aid into Mariupol and civilians out of the city, which has suffered weeks of heavy fighting with dwindling water, food and medical supplies.

“We are running out of adjectives to describe the horrors that residents in Mariupol have suffered,” ICRC spokesperson Ewan Watson said Friday during a U.N. briefing in Geneva. “The situation is horrendous and deteriorating, and it’s now a humanitarian imperative that people be allowed to leave and aid supplies be allowed in.”

He said the group had sent three vehicles toward Mariupol and a frontline between Ukrainian and Russian forces but two trucks carrying supplies for the city were not accompanying them. Dozens of buses organized by Ukrainian authorities to take people out also had not started approaching the dividing line, Watson said.

City authorities said a little while later that the Russians were blocking access to Mariupol and it was too dangerous for people to leave it on their own.

“We do not see a real desire on the part of the Russians and their satellites to provide an opportunity for Mariupol residents to evacuate to territory controlled by Ukraine,” Petro Andryushchenko, an adviser to the mayor of Mariupol, wrote on the Telegram messaging app.

He said Russian forces “are categorically not allowing any humanitarian cargo, even in small amounts, into the city”

On Thursday, Russian forces blocked a 45-bus convoy attempting to evacuate people from Mariupol after the Russian military agreed to a limited cease-fire in the area, and only 631 people were able to leave in private cars, the Ukrainian government said.

Russian forces also seized 14 tons of food and medical supplies trying to make it to Mariupol, Deputy Prime Minister Iryna Vereshchuk said.

The city has been the scene of some of the worst suffering of the war. Tens of thousands of residents managed to leave in the past few weeks through humanitarian corridors, reducing the population from a prewar 430,000 to an estimated 100,000 by last week. But continued Russian attacks have repeatedly thwarted aid and evacuation missions.

In the past few days, the Kremlin, in a seeming shift in its war aims, said that its “main goal” now is gaining complete control of the Donbas, where Mariupol is located. The Donbas is the predominantly Russian-speaking industrial region of eastern Ukraine where Moscow-backed separatists have been battling Ukrainian forces since 2014 and have declared two areas as independent republics.

Western officials said there were growing indications Russia was using its talk of de-escalation in Ukraine as cover to regroup, resupply and redeploy its forces for a stepped-up offensive in the east.

Russian forces have subjected both Chernihiv, a besieged city in northern Ukraine, and the capital of Kyiv to continued air and ground-launched missile strikes despite Moscow saying Tuesday it planned to reduce military activity in those areas.

Elsewhere, Ukrainian forces have retaken the villages of Sloboda and Lukashivka, south of Chernihiv and along one of the main supply routes between the city and Kyiv, according to Britain’s Defense Ministry.

Ukraine has also continued to make successful but limited counterattacks to the east and northeast of Kyiv, the ministry said.

Hours later, Belgorod governor Vyacheslav Gladkov wrote on Telegram early Friday that the fire at the oil depot “occurred as a result of an airstrike from two helicopters of the armed forces of Ukraine, which entered the territory of Russia at a low altitude.”

The depot run by Russian energy giant Rosneft is located about 35 kilometers (21 miles) north of the Ukraine-Russia border.

Separately, Ukraine’s state power company, Energoatom, said Russian troops pulled out of the heavily contaminated Chernobyl nuclear site in northern Ukraine early Friday after receiving “significant doses” of radiation from digging trenches in the exclusion zone around the closed plant.

The International Atomic Energy Agency said it could not independently confirm the exposure claim. Energoatom gave no details on the condition of the soldiers and did not say how many were affected. There was no immediate comment from the Kremlin.

The agency, which is the U.N.’s nuclear watchdog, said it had been informed by Ukraine that Russian forces at Chernobyl had transferred control of the site of the world’s worst nuclear disaster to the Ukrainians in writing.

IAEA Director-General Rafael Grossi wrote on Twitter that he would visit the decommissioned plant as soon as possible and his agency’s “assistance and support” mission to Chernobyl “will be the first in a series of such nuclear safety and security missions to Ukraine.”

Grossi was in the Russian enclave of Kaliningrad Friday for talks with senior officials about nuclear issues in Ukraine. Nine of Ukraine’s 15 operational reactors are currently in use, including two at the Russian-controlled Zaporizhzhya facility, the agency said.

Russian forces seized the Chernobyl site soon after invading Ukraine on Feb. 24, raising fears they would cause damage or disruption that could spread radiation. The workforce there oversees the safe storage of spent fuel rods and the concrete-entombed ruins of the reactor that exploded in 1986.

Five weeks and one day into a conflict that has left thousands dead and driven more than 4 million refugees from Ukraine, there seemed little faith that the two sides would find agreement on their respective demands any time soon.

Russian President Vladimir Putin said conditions weren’t yet “ripe” for a cease-fire and he wasn’t ready for a meeting with Ukrainian President Volodymyr Zelenskyy until the negotiators do more work, Italian Premier Mario Draghi said after a Thursday telephone conversation with the Russian leader.

In his nightly video address late Thursday, Zelenskyy doubted Moscow’s willingness to end the conflict. He warned that Russian withdrawals in the country’s north and center were just a military tactic to build up strength for new attacks in the southeast.

“We know their intentions,” Zelenskyy said. “We know that they are moving away from those areas where we hit them in order to focus on other, very important ones where it may be difficult for us.”