The United States has called for an "immediate cease-fire" in Ukraine to ensure an unimpeded probe into the crash of a Malaysian airliner over eastern Ukraine, amid claims that the aircraft was shot down.
White House spokesman Josh Earnest said in a statement, "We urge all concerned -- Russia, the pro-Russian separatists, and Ukraine -- to support an immediate cease-fire in order to ensure safe and unfettered access to the crash site for international investigators and in order to facilitate the recovery of remains."
The Malaysian Airlines passenger plane with 298 people crashed a in rebel-held area near the Russian border, in what Ukrainian President Petro Poroshenko said was the result of a "terrorist act."
Live Blog: As The Events Unfolded
U.S. President Barack Obama, in calls with Poroshenko and Dutch Prime Minister Mark Rutte, said evidence from the crash site must not be moved.
The White House said Obama told Rutte the United States was prepared to contribute "immediate assistance" for "a prompt, full, credible, and unimpeded international investigation."
The statement said Obama and Rutte also agreed on the need for "immediate access" to the site of the crash in territory held by pro-Russia separatists.
Earlier, Obama told Poroshenko that evidence from the plane must not be taken out of the country until a "thorough and transparent" investigation had taken place.
Obama, who called the incident "a terrible tragedy," assured Poroshenko that U.S. experts will "offer all possible assistance immediately" to investigate what caused the crash.
U.S. officials said the jet was hit by a surface-to-air missile but could not say who launched the attack and from where.
Malaysian Jetliner Goes Down In Eastern Ukraine
The plane was flying from Amsterdam to Kuala Lumpur at an altitude of 10,000 meters. The separatists claimed they could not have shot down the plane at that altitude and blamed the Ukrainian military.
A Ukrainian Interior Ministry adviser, Anton Herashchenko, earlier said the plane had been shot down by a Buk ground-to-air launcher system.
Both Ukrainian and Russian forces have SA-17 systems -- also known as Buk.
Closing Off Crash Site
Separatists had also claimed recently about having acquired Buk systems.
Meanwhile, an OSCE statement said that the separatists have agreed to close off the site of the crash, allow the recovery of bodies, and provide safe access and security to national and international investigators as well as OSCE monitors.
The statement said the deal had been agreed in Kyiv on July 17 after a meeting of the Trilateral Contact Group consisting of Ukraine, Russia, and the OSCE.
The airliner crashed a day after Washington toughened sanctions on Russia, saying Moscow had not done enough to get separatists to lay down their weapons or to stop the flow of weapons and material across the border.
Russian President Vladimir Putin has said Ukraine bore responsibility for the crash.
Putin told a cabinet meeting in Moscow, "This tragedy would not have happened if there was peace in the country, if military operations had not resumed in the south-east of Ukraine."
UN Secretary-General Ban Ki-moon has called for a full international investigation of the disaster, while Britain is seeking an emergency meeting of the UN Security Council to discuss the crisis in eastern Ukraine.
Malaysia's Prime Minister Najib Razak said, "if it transpires that the plane was indeed shot down we insist that the perpetrators must swiftly be brought to justice."
Malaysia Airlines said there were at least 154 Dutch, 27 Australians, 23 Malaysians, 11 Indonesians, six Britons, four Germans, four Belgians, three Phillippinos and one Canadian aboard the Boeing 777 jet.
There were a further 47 passengers whose nationality was not yet known. All flight crew were Malaysian.
It was the second time that a Malaysia Airlines plane was lost in less than six months. Malaysia Airlines Flight 370 disappeared in March while en route from Kuala Lumpur to Beijing.