SHAKHTARSK, Ukraine - The downing of Malaysian airliner MH17 may constitute a war crime, the UN said Monday, as explosions and fighting blocked a new attempt by Dutch and Australian police to reach the crash site in insurgent-held east Ukraine.The unarmed international mission was forced to turn around as heavy bombardment rocked towns close to the site, where the remains of some of the 298 victims still lie more than 10 days after the disaster. Ukraine’s military confirmed that its forces were engaged in fierce battles against rebels nearby, saying its troops had entered into towns including Shakhtarsk, just 10 kilometres (six miles) away. More than 1,100 people have been killed in the violence engulfing east Ukraine in the past three months, the UN said, a toll that does not include the nearly 300 crash victims. UN High Commissioner for Human Rights Navi Pillay condemned the ‘horrendous shooting down’ of the Malaysian passenger jet in 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. The West has accused rebels of shooting down the plane using a missile system which Washington believes was supplied by Russia.But a Dutch-led investigation into the crash has made little headway due to the intensifying fighting in the insurgent-held zone. An AFP reporter in Shakhtarsk said artillery fire could be heard in the town and plumes of black smoke billowed into the sky, while a car was seen driving away with the sign ‘children’ written in red on its front and back. A couple was also seen leaving the town on foot with a young boy, as the woman shouted: ‘Let’s go! Let’s go. The Ukrainian military confirmed that its forces ‘had entered into’ Shakhtarsk and nearby Torez, and that ‘battles were continuing for the complete liberation’ of the town of Snizhne. If Kiev regains control of these locations, it could cut off access to the main rebel bastion Donetsk from the border with Russian, which stands accused by the West of funneling arms to the insurgents.The escalating fighting has led Dutch authorities to conclude that it was unrealistic to send an armed mission to secure the crash site as troops risked getting dragged into the conflict. ‘We concluded with our international partners that there’s a real risk of such an international military mission becoming directly involved in the conflict in Ukraine,’ Dutch Prime Minister Mark Rutte, whose country lost 193 people in the disaster, told journalists.An unarmed team was forced to turn back on Monday after ‘explosions’ in the zone, a Dutch justice ministry spokeswoman said. So far, international monitors have visited the site only sporadically because of security concerns, even though both Kiev forces and pro-Russian separatists had earlier called a truce in the immediate area. Amid international recriminations over the chaos blocking access to the site, both sides in Ukraine’s war traded blame with Kiev accusing the rebels of ‘destroying evidence’ and the insurgents saying Ukraine’s army was targeting civilians.Washington released new photographs to bolster its claim that Russia was now taking a direct role in the conflict by firing into Ukraine, targeting the armed forces. Meanwhile, Russia hit back by demanding the US ‘stop hindering’ the work of monitors trying to check the situation on the ground. The only point both sides appeared to agree on was the need for a ceasefire in east Ukraine, according to the Russian foreign ministry.Fighting continued overnight, as the Ukrainian army appeared to be intensifying their offensive to wrest control of the industrial east. 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.An AFP reporter said bursts of gunfire also rang out in the city centre on Monday morning, a day after fighting claimed 13 lives, including those of two children in the city of Gorlivka, to the north of Donetsk. The military said it is also massing troops around Gorlivka ‘in preparation for liberating it’, after the presidency said Kiev had reclaimed the strategic hill of Savur Mogyla - an ancient holy site and World War II monument some 40 kilometres from the MH17 site. Local authorities in the second main rebel base of Lugansk said that five civilians were killed and 15 injured due to ‘constant firing’ over the past 24 hours. Amid the fighting 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 high commissioner said.In Brussels, the European Union is drafting tougher sanctions against Russia. Action targeting entire economic sectors are being considered, including an arms embargo, while on Tuesday the 28-member bloc is expected to unveil more names of individuals and entities sanctioned. Moscow has blasted the move as ‘irresponsible’, and warned it jeopardised cooperation on security issues.