The International Committee of the Red Cross says it is aware of requests by Ukraine’s U.N. ambassador and others to repatriate the bodies of Russian soldiers killed in action in Ukraine but has no numbers.

Ambassador Sergiy Kyslytsya tweeted Saturday that Ukraine has appealed to the ICRC “to facilitate repatriation of thousands of bodies of Russian soldiers” killed during its invasion of Ukraine. An accompanying chart claimed 3,500 Russian troops have been killed.

Kyslytsya tweeted that parents in Russia should have a chance “to bury them with dignity.” “Don’t let (Russian President Vladimir) Putin hide scale of tragedy,” he urged.

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Laetitia Courtois, ICRC’s permanent observer to the United Nations told The Associated Press Saturday night that the current security situation “is a primary concern and a limitation for our teams on the ground” and “we therefore cannot confirm numbers or other details.”

She said “the ICRC can act as a neutral intermediary” on the return of bodies and other humanitarian issues in conflict, including clarifying the fate of missing persons, reuniting families, and advocating for the protection of detainees “within its possibilities.”

Meanwhile, in Kyiv, the Ukrainian president’s office said Russian forces blew up a gas pipeline in Kharkiv, the country’s second-largest city.

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The State Service of Special Communication and Information Protection warned that the explosion, which it said looked like a mushroom cloud, could cause an “environmental catastrophe” and advised residents to cover their windows with damp cloth or gauze and to drink plenty of fluids.

Ukraine’s top prosecutor, Iryna Venediktova, said the Russian forces have been unable to take Kharkiv, where a fierce battle is underway.

The city of 1.5 million is located 40 kilometers (25 miles) from the Russian border.