SHAKHTARSK - Ukraine's army on Monday seized control of part of the vast site where Malaysian airliner MH17 crashed, insurgents said, as the United Nations announced the downing of the plane could constitute a war crime.
After explosions and fighting blocked a new attempt by Australian and Dutch police to access the east Ukraine crash site, Kiev confirmed that its troops had now entered a string of towns around the scene, including Shakhtarsk, 10 kilometres (six miles) away.
The unarmed international mission was forced to turn back for the second day running before reaching the site, where the remains of some of the 298 victims of the July 17 disaster still lie in the fields.
"It's absolutely unconscionable that they remain out there," said Michael Bociurkiw, a spokesman for monitors from the Organisation for Security and Cooperation in Europe in rebel-held Donetsk.
Alexander Hug, the OSCE mission's deputy head, said: "We have made it clear to everyone involved that tomorrow we absolutely have to have safe access."
But Dutch investigators leading the probe said it was now likely some remains may never be recovered.
"I believe the chances are not very good that we will get it all," Dutch police chief Gerard Bouman told parliament in The Hague.
More than 1,100 people have been killed in the fighting engulfing east Ukraine over the past three months, the United Nations said, a toll that does not include the plane crash victims.
UN High Commissioner for Human Rights Navi Pillay condemned the downing of the jet in what was then rebel-held territory, 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.
The Red Cross has said Ukraine is now in civil war -- a classification that would make parties in the conflict liable to prosecution for war crimes.
Western powers, which have accused Moscow of fanning the rebellion by supplying it with weapons including the missile system allegedly used to shoot down MH17, urged new sanctions against Russia.
But rebel commander Igor Strelkov denied any role in the incident in his first press conference since it happened, held inside the tightly guarded former state security service headquarters in Donetsk.
"I don't know how the plane was downed, by what means. I only know that it was downed and that's it. The only thing I can say is that my men did not down it," said the commander, dressed in army fatigues.
"I did not have in my possession any BUK missile systems. Therefore I could not have given an order to shoot down the Malaysian Airlines plane," he said.
Data from the plane's black boxes analysed as a part of a Dutch-led probe showed the crash was caused by shrapnel from a rocket explosion, Kiev said.
An AFP reporter in the outskirts of Shakhtarsk said artillery fire could be heard in the town and plumes of black smoke billowed into the sky, while a car drove away with the sign "children" written in red.
A couple was seen leaving the town on foot with a young boy, as the woman shouted: "Let's go! Let's go!" A bullet-riddled van lay abandoned by the side of the road, along with an overturned coal truck.
If Kiev manages to cement its latest gains, it could cut off access to Donetsk from Russia, which stands accused by the West of funnelling arms to insurgents.
The rebels did not specify which part of the crash site is now back under Kiev control and there was no confirmation from Ukrainian officials.
"Things are fluid at the moment. Someone who has control of it now may not have control in a few hours," Bociurkiw said.
Andriy Lysenko, Ukraine's military spokesman, claimed that troops were not carrying out any fighting but that "we would occupy (the crash site) once the rebels withdraw".
Washington meanwhile released new photographs to bolster its claim that Russia was now taking a direct role in the conflict by firing into Ukraine.
Russia also said that international monitors would visit its side of the volatile border over the next few days after accusing the United States of "hindering" their work on the ground.
'Both sides using heavy arms'
Local authorities said three civilians were killed and five injured in Donetsk, a city of one million, which has been serving as a base for international monitors and journalists who are travelling regularly to the crash site some 60 kilometres away.
The military said it was also massing troops around key rebel base Gorlivka, 45 kilometres north of Donetsk, "in preparation for liberating it", a day after fighting there claimed 13 lives.
Local officials in the second main rebel city of Lugansk said that five civilians were killed and 15 injured due to fighting over the past 24 hours.
Amid the combat, Pillay warned that 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," the UN high commissioner said, sounding the warning even as Kiev claimed that rebels on Monday fired unguided Grad rockets at residential buildings in Shakhtarsk.