The CIA did not end its programme to support rebels fighting Syrian President Bashar al-Assad in a bid to appease Russia, the head of the US military's special operations said.

General Tony Thomas confirmed that the CIA is shutting down four-year-old operation, which armed and trained rebels, but denied the move was aimed at improving US relations with Assad's ally Russia - a claim made by the Washington Post which first reported the suspension on Wednesday.

It was a "tough, tough decision" but "absolutely not a sop to the Russians," Thomas said at a forum in Aspen, Colorado.

"It was, I think, based on an assessment of the nature of the programme, what we're trying to accomplish, the viability of it going forward," he said on Friday.

The chairman of the Russian upper house of parliament's international affairs committee, Konstantin Kosachev, had welcomed the "long-awaited, excellent" news that the United States was ending the programme.

"This change of affairs will, without any doubt, create additional opportunities for Russian-American collaboration in anti-terrorism in the country," Kosachev said on his Facebook page, earlier this week.

READ MORE: Syria's Bashar al-Assad calls US forces 'invaders'

The CIA declined to comment on Thomas' comments.

Former US President Barack Obama approved the rebel aid programme in 2013 as various opposition groups sought external support in a general uprising against the Assad regime.

Thousands of Syrian anti-government fighters were trained and armed.

But US commitment remained ambiguous amid doubts in some quarters that the rebels could actually manage to depose Assad, and as attention turned to the rising power of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

Support for the programme further eroded last year after the rebels lost the areas they held in the Syrian city of Aleppo, under a Russian-backed government assault.

US President Donald Trump has said the main US focus in Syria should be to eliminate ISIL, rather than opposing the Assad government.

US officials said this week that some of the anti-Assad forces could be absorbed into US military-supported groups fighting ISIL.

Meanwhile, a Syrian military source said Syrian warplanes carried out air raids against ISIL in an area of countryside east of Raqqa near the provincial boundary with Deir Az Zor.

Syrian state TV reported that the attacks "destroyed several bases and vehicles" belonging to the group.

The Syrian Observatory for Human Rights monitoring group said it was the first time in several weeks that the Syrian military had launched an attack in that area, which is close to where US-backed anti-ISIL forces are operating.