Originally Published: 03 AUG 22 23:28 ET

Updated: 04 AUG 22 08:24 ET

By Kaitlan Collins and Paul LeBlanc, CNN

(CNN) -- US officials believe Russia is preparing to falsify evidence to blame Ukrainian forces for last week's deadly blast at the Olenivka prison ahead of visits to the site by outside parties.

An administration official told CNN they expect Russia will falsify evidence, blame Ukrainian forces, and even have "reason to believe that Russia would go so far as to make it appear that Ukrainian HIMARS were to blame before journalists arrive."

The US supplied Ukraine with the HIMARS multiple launch rocket system earlier this year. The US believes Russia has collected large quantities of HIMARS fragments from strikes against its positions in Ukraine and is likely to deliberately place them at sites such as the prison to claim Ukraine was responsible, according to an administration official familiar with the latest information.

Ukrainian President Volodymyr Zelensky said Friday the attack on the prison in separatist-held eastern Ukraine, which resulted in the deaths of at least 50 prisoners, was "a deliberate war crime by the Russians." Russia, meanwhile, blamed Ukraine for the attack.

Kremlin spokesperson Dmitry Peskov on Thursday reiterated the Russian claim that the Ukrainian military was responsible for the attack.

"There is evidence here and there is nothing to hide here," he told CNN. "Moreover, you know that the Russian side proposed to the UN and invited the Red Cross to get acquainted with this evidence on the spot, to conduct the necessary exhaustive investigation."

The International Committee of the Red Cross confirmed Wednesday it had been able to visit the Olenivka facility only once -- in May this year to deliver water tanks. "But we did not have access to POWs held there on an individual basis -- as per ICRC's modalities of work in detention facilities -- and that continues to be the case," it said.

The ICRC added: "Under the Third Geneva Convention, during international armed conflicts, the ICRC must be granted access to all PoWs, wherever they are held. We also have full liberty to choose the places we wish to visit. Since February 2022, our teams have been able to have access to some PoWs, but not all."

The ICRC says it has requested access to the detention center again since the attack last week, but has not received permission from the Russians.

The Olenivka prison near Donetsk has been used to house many of the Ukrainian soldiers who surrendered at the Azovstal plant in Mariupol several months ago. CNN could not independently verify the allegations of either side.

United Nations Secretary-General António Guterres said in a Wednesday news conference that the UN is seeking to establish a fact-finding team to study the attack following requests from both Russia and Ukraine to investigate. He added that the terms of reference for the panel would need to be accepted by Russia and Ukraine before the fact-finding mission would begin.

Video aired on Russian networks and shared on social media channels in Donetsk show extensive destruction to a building and several bodies. CNN was able to geolocate footage of the strike to an industrial area about two miles outside the frontline town of Olenivka.

The Ukrainian military said the explosion took place on the territory of the industrial zone, in a newly-constructed building specially equipped to hold prisoners taken out of Azovstal.

The Ukrainian prosecutor's office has launched an investigation into the strike. In a statement, it said pretrial preliminary data shows "the occupying state struck the territory of penal colony No. 120 in the temporarily occupied Olenivka village of Volnovakha district of Donetsk region."

Donetsk and Luhansk are the two regions that together form Donbas, the eastern part of Ukraine where the conflict between Ukraine and Russian-backed separatists started in 2014.

The area has become the centerpiece of Russian President Vladimir Putin's military ambition in Ukraine after his troops failed to take over Kyiv earlier this year.

The Russian military has kept up a persistent barrage of artillery and missile strikes across the region for several weeks. The Kremlin says the goal of what it calls its "special military operation" is to take control of both Luhansk and Donetsk.

This story has been updated with additional details.