The downing of Malaysian airliner MH17 in eastern Ukraine could amount to a "war crime" according to the UN human rights chief Navi Pillay.
Pro-Russian rebels are being blamed by Ukraine and the West for shooting down the plane using a Buk missile system supplied by Russia. All 209 people on board, most of them Dutch, died.
Moscow and the rebels say Ukrainian forces downed the plane.
"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," the High Commissioner said. "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."
A UN report documenting the deteriorating situation in the Donetsk and Luhansk regions, with armed groups of rebels challenging Kiev's authority, put the number of those killed from mid-April to June as 1,000. Those fleeing the areas of fighting between pro-Russian and Ukrainian forces are more than 100,000.
"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.
The UN monitoring mission deployed in Ukraine listed 812 people who have been abducted or detained by separatists since mid-April.