"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 opposition groups against Syrian army.
Earlier on Friday, Russian Defense Minister Sergei Shoigu suggested the two countries launch joint strikes against al-Nusra Front and other terrorist 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".
Davis said Russia had not made a formal proposal to the United States. "I've only seen the same press reports you have - nothing formal has been presented to us," he told reporters, Al Jazeera reported.
Staffan de Mistura, the United Nations special envoy for Syria, recently estimated that 400,000 have been killed throughout the five years of bloodshed in the country.