UN High Commissioner for Human Rights Navi Pillay condemned the "horrendous shooting down" of the Malaysian passenger jet in rebel-held territory that killed all 298 people on board, and demanded a "thorough, effective, independent and impartial investigation".
"This violation of international law, given the prevailing circumstances, may amount to a war crime," she said in a statement.
"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.
The Red Cross officially said last week that Ukraine is now in civil war -- a classification that would make parties in the conflict liable to prosecution for war crimes.
The UN said that latest figures showed that more than 1,100 people have been killed in fighting on the ground in east Ukraine as both government forces and rebels have increasingly used heavy weapons in built-up areas.
"As of 26 July, at least 1,129 people have been killed and 3,442 wounded," the UN statement said.
The latest toll marks a sharp rise from that given a month ago on June 18, when the UN said at least 356 people had been killed since April.
Pillay described reports of increasingly intense fighting in rebel bastions Donetsk and Lugansk regions as "extremely alarming" and said both sides were "employing heavy weaponry in built-up areas, including artillery, tanks, rockets and missiles."
"Both sides must take great care to prevent more civilians from being killed or injured," Pillay said.
Some 100,000 people have now fled the conflict zone in the east for other areas of Ukraine, the UN said in the report released today.
The report also accused rebels controlling swathes of territory of conducting a brutal "reign of terror" in the areas they control, including the abduction, torture and killing of civilians as the rule of law has collapsed.
"These groups have taken control of Ukrainian territory and inflicted on the populations a reign of intimidation and terror to maintain their position of control," the report said.