DONETSK, Ukraine -- Almost two weeks after Malaysia Airlines Flight 17 was blown out of the sky, the remains of some passengers are feared rotting in the 90-degree (32-degree Celsius) midsummer heat, deepening the frustration of relatives desperate to recover the bodies of their loved ones.

Fighting between Ukrainian forces and separatist rebels has kept away international police charged with securing the site, a sprawling area of farmland and villages. And until it's secured, there is no way for forensic experts to recover the bodies or gather debris for analysis.

Even the rebels -- who initially oversaw the collection of more than 200 of the 298 bodies in a disorganized, widely criticized effort -- have stopped their work, saying attacks from the Ukrainian military have forced them to focus on defending themselves.

It remains unclear exactly how many bodies remain and what condition they are in after being exposed for so long to the elements. Dutch officials are adamant there are still bodies to be recovered, and Prime Minister Mark Rutte has said repeatedly that bringing them back is his government's top priority.

But Dutch officials were gloomy Wednesday about the prospects of reaching the site any time soon.

"We don't expect the security situation to improve enough over the next few days to make this possible," said Pieter-Jaap Aalbersberg, head of the Dutch-led recovery mission.

Two crucial pieces of evidence -- the flight recorders -- have already been retrieved and analyzed, however.

The U.S. and Ukrainian governments say the Boeing 777 was brought down July 17 by a Russian-made missile fired by eastern Ukraine's pro-Moscow separatists. The separatists deny it; Russia denies providing the Buk missile launcher and says the Ukrainian military may have shot the plane down.

After the investigative team's failure to reach the site on Wednesday, the United Nations called on both sides in Ukraine's grinding civil conflict to cease hostilities in the area.

"The families of the victims of this horrific tragedy deserve closure and the world demands answers. International teams must be allowed to conduct their work," U.N. spokesman Stephane Dujarric said.

In their latest attempt to get to the wreckage zone, observers from the Organization for Security and Cooperation in Europe left in two vehicles from the rebel-held city of Donetsk but got only as far as the city's outskirts.

The observers talked with rebels and turned back after being "warned of gunfire on the route and in the surrounding areas," the Dutch said in a statement.

Recent offensives by the Ukrainian army have enabled it to take back swaths of territory from the rebels. But the fighting has edged ever closer to the crash zone.

"We are still waiting and it is a miserable process," said Jasmine Calehr, the grandmother of two Dutch brothers who died in the crash

International observers say wreckage has been cut, moved or otherwise tampered with.

Of the 298 who died, 194 were Dutch citizens, and Ukraine has asked for their government's help in investigating the crash. Thirty-seven were from Australia. A total of 227 coffins have been flown to the Netherlands for identification and investigation.

Despite her mounting frustration, Calehr said she did not want investigators to go unless it was safe.

"Other people are not supposed to risk their lives," she said. "But that there is nobody strong enough to put pressure on a bunch of rebels is very painful."

Ukrainian government security spokesman Andriy Lysenko added to security concerns Wednesday by accusing the separatists of mining the approaches to the area. Even if rebels leave, he said, it will take time to remove the mines and make the area safe for investigators.

Meanwhile, the head of Ukraine's security services revealed what he said was fresh satellite imagery proving Russia had created a major cross-border corridor for the delivery of military equipment to the rebels.

Valentyn Nalyvaichenko said other photographs showed burn marks from rockets fired at Ukrainian troops from a position two kilometres inside Russian territory.