AMSTERDAM — The Dutch government has rejected a call to hand over the investigation into the Malaysia Airlines Flight 17 disaster to the United Nations, saying it's doing the best it can under difficult circumstances.

Flight 17 was shot down July 17 over territory held by pro-Russia rebels in eastern Ukraine, killing all 298 people aboard — most of them Dutch. Hunks of the wreckage arrived Tuesday in the Netherlands by truck.

A law firm representing 20 family members of victims from four countries has criticized the Dutch investigation as too slow and bureaucratic and urged Prime Minister Mark Rutte to turn the job over to the U.N.

The government's top security official replied Tuesday that investigators that are doing their best while facing a "complex geopolitical situation in a conflict zone."

"We are doing what we can, and we do it every day anew," Security and Counterterrorism Coordinator Dick Schoof said in a statement.

International teams seeking to retrieve human remains and salvage evidence have had difficulty reaching the crash site due to clashes between Ukrainian and rebel forces. Six victims have yet to be identified.

The Dutch Safety Board, which is leading the investigation, said in a preliminary report in September that the plane was brought down by "high-energy objects from outside the aircraft."

That vague formulation has left the door open for confusion. A high-ranking rebel officer has acknowledged that rebels shot down the plane with a ground-to-air missile after mistaking it for a Ukrainian military plane. Russian media, however, claim the plane was shot down by a Ukrainian jet.

The Safety Board's final report may rule out one or the other scenario but it will not seek to attribute responsibility.

Dutch prosecutors, meanwhile, are coordinating an international criminal investigation into the downing but have yet to name any suspects or say when or how charges might be brought.

"For the Cabinet, it is of the utmost importance that the independent investigations indisputably determine what happened and that no one can dispute the conclusions ... or say that the Netherlands as leader gave cause to doubt the independence of the investigation," Schoof wrote.

Bob van der Goen, founder of the law firm Van Der Goen Advocaten, told The Associated Press the Dutch performance so far "has been terribly amateurish," citing investigators' late arrival to the crash site and a failure to interview potential witnesses in Ukraine in a timely manner.

"I can understand keeping the investigation confidential so that you don't release partial results," he said. "What I don't understand is secrecy about what it is you're undertaking, what the procedure is."

Safety Board chief Tjibbe Joustra said investigators will further photograph, scan and categorize pieces of the wreckage now at the southern Gilze-Rijen military base, then later reassemble part of the Boeing 777 in a hangar.

"It will take several months before we have a reconstruction of parts of the aircraft," he said.