STEVE CANNANE, PRESENTER: Australia's head of the MH17 recovery mission, Angus Houston, says it may be months before investigators can return to the crash site in eastern Ukraine.

Angus Houston said increased fighting between Ukrainian forces and pro-Russian rebels posed an extreme danger to the team of 70 Australian, Dutch and Malaysian experts.

Shooting near the crash zone overnight forced the investigators to shelter, providing just one incident too many for the officials.

Europe correspondent Philip Williams reports from Kiev.

PHILIP WILLIAMS, REPORTER: It's been a frustrating few days, but despite the real threat searching a war zone, the leaders of this mission were determined they would see it through, that is, until today.

ANGUS HOUSTON, SPECIAL ENVOY: As we all know, it's a very complex situation and that complex situation was being reflected on increasing difficulties in getting access to key parts of the crash site. So, we're getting to the point where people are going out there, unarmed, with what appears to be a worsening situation around them and I think it got to the point where - diminishing returns. We weren't getting to find anything. The last three days didn't find any human remains. The security situation was fluid and we were worried that at some stage, there might be a miscalculation or a mistake and we would be caught out.

PHILIP WILLIAMS: The plan is to return once it's safe and complete the task, although with all but a small contingent leaving the country, and soon, it doesn't look like the recovery process will resume in the near future.

ANGUS HOUSTON: We'll come back later. Now, we think we've got most of the remains, but, we'd like to go over the ground at a time when the vegetation is more helpful, perhaps in the autumn when - before the snows come, when all the vegetation disappears, and also when the security situation improves and our experts can actually do their work without worrying about the security situation.

PHILIP WILLIAMS: Fighting between the rebels and Ukrainian forces has grown even more intense in recent days, with the rebel-held cities of Luhansk and Donetsk virtually surrounded.

NATO says it fears a huge buildup of Russian troops on the border with Ukraine could be the precursor to an invasion under the guise of a peace-keeping mission.

Certainly the Polish Prime Minister, Donald Tusk, thinks there are real dangers ahead.

DONALD TUSK, POLISH PRIME MINISTER (voiceover translation): We have reasons to suspect from information received in the last several hours that the risk of a direct Russian intervention is certainly higher than it was several days ago.

PHILIP WILLIAMS: That risk may have also played on the minds of politicians and planners from Australia, the Netherlands and Malaysia.

In Kiev, Australians held their memorial for those who died. The difficult task of ensuring all remains make it home will have to wait.

Philip Williams, Lateline.