Intensifying clashes close to the crash site of Malaysia Airlines flight MH17 have prevented international investigators visiting the scene, as Kiev’s forces fight pro-Russian rebels for control of strategic towns in eastern Ukraine.

The investigators were thwarted in their attempt to examine the crash zone and wreckage yesterday, as a senior United Nations official said the downing of the Boeing 777 could be a war crime.

Kiev and western allies say the rebels downed the plane with a high-tech missile provided by Russia – a claim the insurgents and Moscow deny. Ukrainian security council spokesman Andriy Lysenko said the plane crashed due to a “massive explosive decompression” after being struck by shrapnel from a rocket.

British experts who downloaded data from the plane’s flight recorders declined to comment but said they had given the retrieved data to an investigative team led by the Netherlands, whose citizens accounted for two-thirds of the crash victims.

Separatist militia Ukrainian troops are fighting separatist militia in and around Torez, Shakhtyorsk and Snezhnoe, towns within about 30

km of the fields and villages over which the aircraft flying from Amsterdam to Kuala Lumpur disintegrated on July 17th, killing all 298 people on board.

The towns lie along a key road route by which the rebels – who reject Kiev’s new pro-EU government and want eastern Ukraine to be ruled by Moscow – allegedly receive large amounts of weapons and reinforcements from Russia.

The Organisation for Security and Co-operation in Europe said its experts and investigators from the Netherlands and Australia had been unable to reach the crash site for a second day due to “security reasons”.

Sporadic shelling and fighting also continued close to the industrial region’s main city, Donetsk, where rebels have dug trenches and set up mortar positions and vowed to create “another Stalingrad” if stormed by Kiev’s troops.

The military also said it was massing around Horlivka, a rebel stronghold north of Donetsk, where shelling killed 13 civilians on Sunday.

Rebels and government troops accuse each other of firing heavy artillery at residential areas, but US-based Human Rights Watch said last week that state forces were almost certainly responsible for most deadly rocket fire around Donetsk.

Analysts say Ukraine’s impoverished army relies on shelling because it does not have enough troops capable of rooting out rebels from urban areas, from where they are accused of firing on servicemen from or around civilian buildings. Navi Pillay, the UN’s top human rights official, said yesterday at least 1,129 people had been killed in eastern Ukraine since fighting began in April.

Over the same period, the report said, at least 3,442 people had been injured and more than 100,000 people had left their homes.

Ms Pillay called for a full, independent investigation into the fate of flight MH17, saying: “This violation of international law, given the prevailing circumstances, may amount to a war crime.” She added: “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.”