DONETSK, Ukraine - Thirteen people including two children were killed Sunday in fierce clashes in the key insurgent holdout of Gorlivka in eastern Ukraine where government troops are battling rebels, local authorities said.

‘As a result of military actions in Gorlivka 13 people have died, among them two children of one and five,’ the Donetsk regional administration said in a statement, adding that operations by medical teams in the crucial insurgent base, a city of some 250,000 around 45 kilometres (28 miles) north of Donetsk, were hampered by ‘constant shooting’.

An eyewitness told AFP that mortar fire rained down on the centre of the city after rebels warned that there would be ‘intense firing’. ‘In the park I saw a man, a woman and a child dead on the ground,’ local resident Ludmila said. ‘Near the railway station a shop was destroyed, while the bus station was also on fire and dead people were lying around,’ she said.

Ukraine’s military accused insurgent fighters of firing Grad rockets at residential blocks in the city ‘aiming to bring discredit to the Ukrainian army and frighten the non-combattants’. A rebel commander from the self-proclaimed Donetsk People’s Republic told a press conference that the situation in Gorlivka was ‘fine for the moment’. Seized by separatists in April, Gorlivka has been the headquarters for a hard core of insurgent militants, including an important rebel commander linked by Ukrainian security services to the downing of Malaysia Airlines flight MH17. Moreover, Dutch experts have cancelled plans to head to the site of the downed Malaysia Airlines flight MH17, international officials say.

Fighting between pro-Russia separatists and government troops in the area has prevented access to the site, they add. Earlier, Malaysia said it had struck a deal with the rebels to allow international police at the site. MH17 crashed on 17 July, killing all 298 people on board. The rebels have been accused of shooting it down.

Russia has suggested the plane could have been shot down by the Ukrainian military - an allegation Ukraine denies. The investigators, who are currently in Donetsk, have struggled to gain access to the rebel-controlled crash site. ‘There is fighting going on. We can’t take the risk,’ said Alexander Hug, of the Organization for Security and Co-operation in Europe (OSCE).

‘The security situation on the way to the site and on the site itself is unacceptable for our unarmed observer mission,’ he added. The eastern regions of Donetsk and Luhansk have been gripped by heavy fighting as government forces try to retake rebel strongholds. In the latest fighting in Donetsk, at least 13 civilians were reported to have been killed in Horlivka, north of the regional capital, as troops try to seize the town.

Shelling was also reported close to the MH17 crash site, near the town of Grabove, on Sunday. Rebels have prevented journalists going to the site and Ukrainian government forces are said to be nearby, says the BBC’s Tom Burridge, in eastern Ukraine.

A total of 227 coffins containing the remains of the victims have been sent for identification to the Netherlands, which is leading the crash investigation. The first MH17 victim has been identified, though officials did not reveal any details. Officials say the exact number of bodies already collected will be determined only after forensic experts have completed their examination.