UNITED NATIONS (AFP) - The Malaysia Airlines jet that crashed in Ukraine, killing 298 people, was "likely downed" by a surface-to-air missile launched from a separatist-held area, a US envoy said Friday.

Flight MH17 was "likely downed by a surface-to-air missile, SA-11, operating from a separatist-held location in eastern Ukraine," Samantha Power, Washington's ambassador to the United Nations, told an emergency meeting of the Security Council.

British ambassador Mark Lyall Grant also said early indications showed the flight was brought down by a surface-to-air missile.

Britain and the United States said the tragedy should serve as a wake-up call for Russia to stop supporting and equipping rebels fighting in eastern Ukraine.

Power said given the technical complexity of missile systems in the hands of separatist rebels, the United States "cannot rule out technical assistance from Russian personnel" in their use.

"The context for yesterday's horror is clear. Separatist forces backed by the Russian government continue to destabilise Ukraine," Power told the Council.

Britain and the United States demanded an immediate ceasefire to allow investigators to access the crash site and said the world must stop at nothing to bring those responsible to justice.

"This tragedy only underscores the urgency and determination with which we insist that Russia immediately take concrete steps to de-escalate the situation in Ukraine, support a sustainable ceasefire and following the path toward peace," Power said.

"This war can be ended. Russia can end this war. Russia must end this war," she added.