Navi Pillay, the United Nations High Commissioner for Human Rights, said Monday that the downing of Malaysia Airlines Flight MH17 could possibly be a war crime, and called for a detailed investigation.

In a statement released by UNHCR Monday, Pillay asked for a thorough investigation into the crash of Flight MH17, which is believed to have been brought down by a surface-to-air missile while it was flying over eastern Ukraine, which has been rocked by months of strife between pro-Moscow rebels and Ukraine's government forces. The UNHCR estimated that at least 1,129 people have been killed in the ongoing violence, and Pillay urged the warring sides to restore peace to the region.

“The horrendous shooting down of the Malaysian Airlines plane on 17 July came just after the cut-off date of this report (on the methods used to treat hostages),” Pillay said, in the statement, adding: “This violation of international law, given the prevailing circumstances, may amount to a war crime. It is imperative that a prompt, thorough, effective, independent and impartial investigation be conducted into this event.”

Citing Ukrainian government estimates, at least 8 billion Ukrainian Hryvnia ($750 million) are required to rebuild Ukraine's east, the statement said.

Although a formal investigation into the crash of Flight MH17 has not yet begun, a team of Dutch experts is attempting to reach the site of the plane's crash, which killed all 298 people on board.  

The U.N. is also increasingly concerned about an increasing amount of arms and ammunition being found in the eastern Ukrainian regions of Donetsk and Luhansk, which Kiev claims are being supplied by Russian forces. Moscow has denied the accusations and has said that it is taking steps to stabilize the situation in the country.

“There is an increase in the use of heavy weaponry in areas that are basically surrounded by public buildings," Gianni Magazzeni, head of the U.N. office's branch that oversees Ukraine, said, according to Associated Press, adding: "All international law needs to be applied and fully respected."

A U.N. report on abductions, torture and other acts of violence in Ukraine, released Monday, also talked about the growing tension in Crimea, and warned that the economic impact of the violence in the country's east would also be felt across the region.

“I would like to stress to all those involved in the conflict, including foreign fighters, that every effort will be made to ensure that anyone committing serious violations of international law including war crimes will be brought to justice, no matter who they are,” Pillay said, according to the statement, adding: “I urge all sides to bring to an end the rule of the gun and restore respect for the rule of law and human rights.”