Top US diplomat John Kerry told his Russian counterpart Saturday that Washington was "deeply concerned" that international investigators were being denied access to the Malaysian jet`s crash site in Ukraine.
President Barack Obama and other world leaders have expressed outrage and demanded Russia`s full cooperation with what is becoming a monumentally challenging probe into the shooting down of a Kuala Lumpur-bound Malaysia Airlines flight from Amsterdam with 298 people from a dozen countries on board.
In a telephone call, Kerry told Russian Foreign Minister Sergei Lavrov that "the United States remains deeply concerned that for the second day in a row, OSCE monitors and international investigators were denied proper access to the crash site," the State Department said.
"The United States is also very concerned about reports that the remains of some victims and debris from the site are being tampered with or inappropriately removed from the site."
Organisation for Security and Cooperation in Europe spokesman Michael Bociurkiw said earlier that the Vienna-based group`s monitoring team on the ground had been "unable today, for the second day, to gain any answers" about the fate of the plane`s critical black box flight data recorders.
In Moscow`s account of the Kerry-Lavrov conversation, it said it had demanded that "material evidence, including black boxes" must be immediately handed over to inspectors.
Gunmen backed up by muscular diplomatic support from the Kremlin have shown few signs of being ready to cooperate with an investigation that could blame them for blowing apart the Boeing 777 jet.
Kiev said armed fighters were hours away from loading vital clues aboard trucks that would be rushed across the Russian border before a full team of experts inspected the expansive site where remains of flight MH17 hit the ground.
Kerry urged Russia, in his call with Lavrov, to take "immediate and clear actions" to reduce tensions in neighboring Ukraine that have pushed the country into an escalating civil war and East-West standoff.
Those steps include to "call on pro-Russia separatists in eastern Ukraine to lay down arms, release all hostages and engage in a political dialogue toward peace with the Ukrainian government; to halt the flow of weapons and fighters into eastern Ukraine and to allow OSCE observers to help secure the border," the State Department added.
"The secretary (Kerry) particularly stressed the international call for investigators to receive full, immediate and unfettered access to the Malaysia Airlines Flight 17 crash site."
Earlier, US Defense Secretary Chuck Hagel spoke by phone with Dutch Defense Minister Jeanine Hennis-Plasschaert, whose country lost 192 citizens aboard Flight MH17.
The pair stressed "the difficulty investigators are experiencing in gaining unimpeded and secure access to the crash site," Pentagon spokesman Rear Admiral John Kirby said in a statement.
Hennis-Plasschaert "highlighted the Netherlands` desire to have the victims returned to their families as soon as possible, balanced with the need to support and complete a credible investigation," Kirby added.