Dutch and Australian police who were planning to head to the crash site of Malaysian flight MH17 on Sunday have had to scrap their trip over security concerns, a OSCE spokesman said.
"There is fighting going on. We can't take the risk. The security situation on the way to the site and on the site itself is unacceptable for our unarmed observer mission," said Alexander Hug, deputy chief monitor of the Organization for Security and Cooperation in Europe's special mission in Ukraine.
Earlier on Sunday, Dutch police had set off for the scene of the Malaysian plane disaster in eastern Ukraine to secure the site, which investigators complain has already been compromised.
Read: Rebels to let Malaysian police secure MH17 site
A dozen Dutch policemen in armored SUVs set off for the site late Sunday morning. Rebel interference and security concerns have limited international investigators' access to the area, which observers say is still strewn with remains.
Australian prime minister Tony Abbott announced on Sunday that unarmed Australian police would be sent to the crash site as part of a Dutch-led police force to secure the area and help recover victims' remains.
Malaysian Airlines Flight 17 was shot down with a surface-to-air missile over a part of eastern Ukraine controlled by pro-Russian separatists last week, killing nearly 300 people on board. U.S. and Ukrainian officials say it was shot down by a missile from rebel territory, most likely by mistake.
Concerns about the integrity of the site were raised further on Saturday when an Australian couple visited the wreckage-strewn fields outside the village of Hrabove and even sat down on part of the debris.
Flights from Ukraine to the Netherlands have taken 227 coffins containing victims of the plane disaster. Officials say the exact number of people held in the coffins still needs to be determined by forensic experts in the Netherlands.
The disaster prompted some expectation in the West that Russia would scale back its involvement in the uprising in Ukraine's east, but ten days later the opposite seems to be the case.
Russia launched artillery attacks from its soil into Ukraine on Friday, while the United States said it has seen powerful rocket systems moving closer to the Ukraine border.
Ukrainian officials said on Saturday that their forces advanced to the outskirts of the regional capital Donetsk as they try to retake the stronghold held for months by pro-Russia rebels.