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The Ukrainian military has launched an effort to take control over the Malaysia Airlines Flight 17 crash site in the eastern part of the country, the New York Times reports. The move comes shortly after the Malaysian government made a deal with pro-Russian separatists to allow an international police team access to the field, hampering continued attempts to secure the site from rebels.

A police team trying to reach the Malaysia Airlines Flight 17 crash site in eastern Ukraine turned back on Monday after hearing explosions and gunfire in the town of Shakhtarsk, about 20 miles from the field, AP reports. The group, including officials from the Organization for Security and Cooperation in Europe (OSCE), as well as Dutch and Australian police and forensic officials, returned to Donetsk.

The team was traveling to the area to secure the wreckage site from pro-Russian rebel forces and clean up remaining debris from the Boeing 777, which was shot down on July 17 by a suspected missile, killing 298 passengers and crew.

United Nations human rights commissioner Navi Pillay said in a report released Monday that the downing of the plane may constitute a war crime. Pillay also said that more than 1,129 people have been killed since the fighting began in April, a death toll from the "extremely alarming" clashes. At least 3,442 have been wounded and more than 100,000 displaced.

Ukrainian troops have reportedly entered Shakhtarsk to push back against separatists, who currently control the town's checkpoints, according to the Defense Ministry. Other towns surrounding the crash site are also experiencing fighting, the ministry said.

Ukraine and the U.S. have both accused rebels of tampering with evidence at the crash site as the investigation continues into who shot down the plane. New evidence released last week indicated that Russia was not directly involved, but may have "created the conditions" for the disaster to occur, according to U.S. officials.

Russian foreign minister Sergey Lavrov called on the U.S. government to release any evidence it had to back up its position that separatists had received the missile from Russia.

“We do not understand why the Americans, who say that they have strong evidence to support their accusation, why they do not show that evidence,” Lavrov said at a news conference.

Andriy Lysenko, a spokesman for Ukraine’s National Security and Defense Council, said that an analysis of the flight data recorders showed that the plane was brought down from multiple shrapnel punctures from a rocket blast, which had caused “massive explosive decompression.” But it is unclear whether the analysis is accurate, as the data recorders are currently being examined in Britain.

The U.N. report also described an increase in human rights abuses in eastern Ukraine, including abductions, torture, and killing of journalists.

“The reports of increasingly intense fighting in the Donetsk and Luhansk regions are extremely alarming, with both sides employing heavy weaponry in built-up areas, including artillery, tanks, rockets and missiles," Pillay said. "Both sides must take great care to prevent more civilians from being killed or injured. Already increasing numbers of people are being killed with serious damage to civilian infrastructure, which – depending on circumstances – could amount to violations of international humanitarian law. The fighting must stop."