UN Secretary-General Ban Ki-moon has called on Kiev and pro-Russian rebels to immediately stop fighting, to allow investigators to reach the crash site of the downed Malaysian plane.

“The families of this horrific tragedy deserve closure and the world demands answers; international teams must be allowed to conduct their work,” said Ban in a statement released on Wednesday by his spokesman.

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The appeal came after investigators and forensic teams from the Netherlands, Australia and Malaysia were forced to turn back for the fourth consecutive day, due to fighting between Ukrainian forces and separatist fighters in east Ukraine.

Saying he was “deeply disturbed” by the developments, Ban called “on all parties to immediately halt hostilities in the proximity of the crash site so as to allow the international teams unimpeded access to the site”.

“There are victims’ remains yet to be found,” Ban said, and “key pieces of evidence remain at the site”.

More than 200 bodies from the 298 victims of the July 17 crash have been recovered from the site and sent to the Netherlands, but there have been concerns of evidence tampering.

The UN Security Council last week adopted an Australian-backed resolution condemning the downing of the passenger plane, demanding that all military operations cease and calling for a full investigation.

Russia and Ukraine have traded accusations over the fighting in east Ukraine, with Kiev accusing Moscow of backing the separatist rebels and most likely providing the weaponry that brought down the passenger plane.

Kiev said its troops were pressing on with an offensive to take back towns near the downed jet, with a “mopping up” operation ongoing in the town of Ilovaysk, some 40 kilometres away.

The army said troops had also taken the town of Avdiyivka, a dozen kilometres to the north of Donetsk.

Fighting also raged around another insurgent stronghold, Lugansk, where local authorities said one civilian died and 10 were injured in the past 24 hours.