Dutch experts have cancelled plans to head to the site of the downed Malaysia Airlines flight MH17 in eastern Ukraine, 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.
Malaysian Prime Minister Najib Razak said on Sunday that he had reached an agreement with Ukrainian separatist leader Aleksander Borodai to allow international police access to the site in order to “provide protection for international crash investigators”.
Australian Prime Minister Tony Abbott said that Australian police officers would also be deployed as part of the Dutch-led mission.
“Our objective is to get in, get cracking and to get out… This is a risky mission, no doubt about that,” he added.
Dutch officials have signalled that they will wait until Ukraine holds a parliamentary vote to approve the presence of an international police force in Grabove before deploying their own officers, our correspondent says.
The US says it believes rebels shot down the passenger jet with a Russian-provided SA-11 Buk surface-to-air missile, probably by mistake.
Russia has frequently denied sending heavy weapons into Ukraine but rebel leaders have given conflicting accounts of whether they had control of a Buk launcher at the time the plane was downed.
In a new development, Russia says it has set up its own team of experts to investigate the plane crash, according to RIA Novosti agency.
Malaysia Airlines commercial director Hugh Dunleavy has called for the formation of “one body to be the arbiter of where we can fly” in the wake of the MH17 disaster.
Writing for the Sunday Telegraph, Mr Dunleavy says “this tragedy has taught us that despite following the guidelines and advice set out by the governing bodies, the skies above certain territories are simply not safe”.