The United States has rejected a Russian proposal to carry out joint air strikes against armed opposition groups in Syria.

"We do not collaborate or coordinate with the Russians on any operations in Syria," Pentagon spokesman Navy Captain Jeff Davis told reporters on Friday, explaining that Washington and Moscow have separate military objectives.

The US and Russia have been carrying out separate bombing campaigns in Syria, where a five-year civil war has pitted armed rebel groups against government forces loyal to President Bashar al-Assad.

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"Russian operations are supporting and enabling the Assad regime and our focus is solely on degrading and defeating ISIL," Davis said, referring to the Islamic State of Iraq and the Levant group (also known as ISIS).

Earlier on Friday, Russian Defence Minister Sergey Shoygu suggested the two countries launch joint strikes against al-Nusra Front and other armed groups that do not support a frail truce brokered by Moscow and Washington in February.

He also proposed that joint strikes be made against "convoys containing weapons and ammunition, armed units that illegally cross the Syrian-Turkish border".

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Davis said Russia had not made a formal proposal to the US. "I've only seen the same press reports you have - nothing formal has been presented to us," he told reporters.

The Syrian conflict started as a largely unarmed uprising in March 2011, but it quickly morphed into a full-on civil war that has left millions displaced and hundreds of thousands dead.

Staffan de Mistura, the United Nations special envoy for Syria, recently estimated that 400,000 have been killed throughout the five years of bloodshed.