THE HAGUE (AFP) - The "complex" situation in the eastern Ukraine means investigators will miss the deadline to file a preliminary report into the cause of flight MH17's crash, the Dutch team leading the probe said on Wednesday.

International regulations require investigators to file an initial report with the UN's International Civil Aviation Authority within 30 days of an accident.

"We won't make it in 30 days. The preliminary safety report will take a while still," said Sara Vernooij, spokeswoman of the Dutch Safety Board (OVV), which is leading the international investigation.

"The reason for the delay is because it's a very complex situation in Ukraine right now," she told AFP.

The probe into downed Malaysia Airlines flight MH17 continued under difficult conditions on Wednesday.

A total 298 passengers and crew were killed when the Boeing 777 jet flying from Amsterdam to Kuala Lumpur was blown out of the sky almost three weeks ago.

The United States says insurgents shot down the plane with a surface-to-air missile likely supplied by Russia, but Moscow and the rebels blame the Ukrainian military.

The aircraft's black boxes have been shipped to Farnborough in Britain to be examined by the Air Accidents Investigation Branch.

OVV investigators have not yet been to the crash site, but "we still want to go," said Vernooij's colleague, Wim van der Weegen.

Asked when a preliminary report could be expected, Vernooij said the team "needed more time." "It still could take several weeks," she said.