Fighting around the site of the downed Malaysia Airlines Flight MH17 prevented a visit by international experts on Sunday, although Malaysia said separatists had agreed to allow in international police and investigators.
The Ukrainian government said its forces were advancing toward the crash site to try to free it from the rebels, who have impeded the work of international monitors and whom Kiev accuses of tampering with evidence pointing to who shot down the airliner.
"All our troops are aiming to get there and liberate this territory so that we can guarantee that international experts can carry out a 100 percent investigation of the site and get all proof needed to deduce the real reason for this tragedy," said Andriy Lysenko, a spokesman for Ukraine's Security Council.
International monitors said the fighting itself could affect the crash site, underlining the growing complexity of trying to establish who shot down the aircraft.
In Donetsk, Alexander Hug, deputy head for the Organization for Security and Cooperation in Europe's monitoring mission in Ukraine, said monitors would not visit the site on Sunday.
"The situation on the ground appears to be unsafe. … We therefore decided to deploy tomorrow morning," Hug, flanked by Dutch and Australian experts, told reporters.
"Fighting in the area will most likely affect [the] crash site," he said.