Pillay, the U.N.'s top human rights official, called for a thorough investigation into the violation of international law that occurred when the flight was shot down with a surface-to-air missile over a part of eastern Ukraine controlled by pro-Russian separatists on July 17, killing all 298 people on board.

Pillay's comments coincided with a new report by her office that says at least 1,129 people had been killed and 3,442 wounded in Ukraine's fighting as of Saturday, and more than 100,000 have fled the violence since April.

"This violation of international law, given the prevailing circumstances, may amount to a war crime," Pillay said of the downed jetliner, which U.S. and Ukrainian officials say was shot down by a missile from rebel territory, most likely by mistake.

"It is imperative that a prompt, thorough, effective, independent and impartial investigation be conducted into this event," she said.

Fighting over the weekend prevented a team of Dutch and Australian police officers from visiting the crash site to start searching for evidence and the remaining bodies. The Dutch government said a team of 26 forensic experts left Donetsk for the crash site on Monday.

A full-fledged investigation still has not begun at the crash site. Some bodies are still unrecovered and the site has been forensically compromised.

The report by the U.N.'s team of 39 field monitors in Ukraine says there has been an alarming buildup of heavy weaponry in civilian areas of Donetsk and Luhansk — including artillery, tanks, rockets and missiles that are being used to inflict increasing casualties and damage to civilian infrastructure.

The report says such attacks could amount to violations of international humanitarian law.

Gianni Magazzeni, head of the U.N. office's branch that oversees Ukraine, told reporters in Geneva that all governments must respect "the presumption of innocence of civilians."

"There is an increase in the use of heavy weaponry in areas that are basically surrounded by public buildings," he said. "All international law needs to be applied and fully respected."