The downing of the Malaysia Airlines jet in Ukraine may be a "war crime", the UN's human rights chief says.
Pro-Russia Ukrainian rebels and the Ukrainian authorities have accused each other of shooting down flight MH17.
A Ukrainian official said on Monday that MH17's data recorders show a "massive explosive decompression" caused by a rocket.
Meanwhile, heavy fighting has again prevented international police from reaching the crash site.
Ukraine's military said it was battling separatists for control of several towns near the site in eastern Ukraine.
All 298 people on board the airliner - mostly Dutch - died on 17 July.
International police want to help secure the huge site so that plane wreckage and human remains can be examined by crash experts.
Most of the bodies have been removed, many of them repatriated to the Netherlands.
"This violation of international law, given the prevailing circumstances, may amount to a war crime," Navi Pillay, the UN High Commissioner for Human Rights, said of the downing of MH17.
"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," Ms Pillay said.
Ms Pillay spoke as the latest UN report on Ukraine suggested at least 1,129 people have been killed and 3,442 wounded in the Ukraine conflict since mid-April.
The conflict has displaced more than 200,000 people, many of whom have fled east to neighbouring Russia.
Pro-Russian rebels have been accused of attacking the plane with a missile fired from a Buk system, also known to Nato as an SA-11 Gadfly. The missile is designed to explode close to an aircraft, spraying it with shrapnel.
Flight MH17's two black boxes are currently being analysed by aviation experts in the UK.
Russian Foreign Minister Sergei Lavrov said he hoped international monitors would be able to deploy on the Ukraine-Russia border in the next few days.
In a statement on Monday, the Ukrainian military said it had "entered" the towns of Shakhtarsk and Torez and was working to seize control of Pervomaysk and Snizhne - all close to the crash site of MH17.
A separatist leader in Donetsk, Vladimir Antyufeev, told the AFP news agency that the Ukrainian army had taken over part of the crash site.
A team of Australian and Dutch police and forensic experts abandoned attempts to reach the site on Monday, blaming the security situation in the area. It was the second failed attempt in as many days.
"We are sick and tired of being interrupted by gunfights despite the fact that we have agreed that there should be a ceasefire," said Alexander Hug, an official from the OSCE mission to Ukraine.
The army is also trying to take control of two main roads in eastern Ukraine, which the government in Kiev believes to be vital supply lines from Russia for rebel forces in Donetsk.
In the past 24 hours there has been heavy artillery fire at the city of Horlivka, where several civilians were killed.
In Donetsk at least three people died in shelling, the municipal authorities say. And there are reports of civilian casualties in the shelling of Luhansk, which is also held by the rebels.
The US has produced what it calls satellite evidence that rockets have been fired at Ukrainian forces from Russian soil.
Russia denies that any of its forces are helping the rebels.