World leaders have demanded an international investigation into the shooting down of a Malaysian airliner with 298 people on board, after Ukraine denied deploying missiles during fighting with pro-Russian rebels in eastern Ukraine.

The US told an emergency UN Security Council meeting on Friday that a "full, credible and unimpeded international investigation must be held immediately" and indicated that it suspected that Russian personnel had helped separatist rebels to fire a missile that brought down the plane on Thursday.

"Because of the technical complexity of the SA-11 it is unlikely that the separatists could effectively operate the system without assistance from knowledgeable personnel, thus we cannot rule out technical assistance from Russian personnel in operating the system," Samantha Power, US ambassador to the UN, said.

German Chancellor Angela Merkel earlier called for a ceasefire to allow access to the MH17 crash site.

"There are many indications that the plane was shot down, so we have to take things very seriously," Merkel said.

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Both sides later on Friday agreed to establish a corridor through the battlefield to allow international investigators in.

Earlier, Ukraine's government said it had not used anti-aircraft missiles in its fight with pro-Russia rebels in the east of the country.

"All missiles that are in our armoury, not one of them has been used," Andriy Lysenko, spokesman for Ukraine's Security Council, said.

Bohdan Senyk, a spokesman for the Defence Ministry, said the passenger jet was out of range of the Ukrainian army's anti-aircraft missile systems: "Anti-aircraft missiles have not been deployed during the anti-terrorist operation ... they are all in place."

Russia's Defence Ministry said on Friday it had picked up radar facility activity from a Ukrainian Buk missile system on Thursday.

Ukrainian President Petro Poroshenko, who had stepped up an offensive in the east, spoke to US President Barack Obama and sought to rally world opinion behind his cause.

"The external aggression against Ukraine is not just our problem but a threat to European and global security," he said.

Meanwhile, pro-Russian separatists told the Organisation for Security and Co-operation in Europe (OSCE), a security and rights body, they would ensure safe access for international experts visiting the scene.

The plane crashed about 40km from the border with Russia near the regional capital of Donetsk, an area that is a stronghold of rebels who have been fighting Ukrainian government forces.

Leaders of the rebels' self-proclaimed Donetsk People's Republic denied any involvement and said a Ukrainian air force jet had brought down the intercontinental flight.

Ukraine on Friday closed the air space over the east of the country.

Source: Agencies