Kiev declared Thursday as the "day of the Organisation for Security and Cooperation in Europe (OSCE)" upon the request of UN Secretary General Ban Ki-moon to facilitate work at the crash site.
The UN chief Wednesday harshly criticised the obstacles faced by international forensic investigators who travelled to Ukraine to investigate the downing of the Malaysian plane.
However, Ukrainian authorities accused the rebels of breaking the ceasefire and firing Grad missiles at dawn Thursday on the town of Pobednoye, near Lugansk.
The separatists, who have lost ground to government troops in the past weeks, argued they had not been informed about the ceasefire.
"We have not heard anything about this and it seems Kiev neither", a spokesperson for the militias told the Russian news agency RIA Novosti, adding that fighting between the two sides continued even on the outskirts of the town of Shakhtiorsk, one of the closest to the area where the wreckage of the Boeing 777 lies.
The press chief of the self-proclaimed People's Republic of Lugansk, Vladimir Inogorodtsev, claimed that "saboteurs" fighting on the Ukrainian side had fired mortar shells on residential areas in Lugansk.
Malaysia Airlines Flight MH17, while flying from Amsterdam to Kuala Lumpur, crashed July 17 in the Donetsk region of eastern Ukraine, killing all 283 passengers and 15 crew on board.
Reports indicated that the Boeing 777 crashed on being hit by a missile. US President Barack Obama said initial investigations showed that the missile was fired from an area in Ukraine controlled by anti-Kiev militants.