The US-led coalition attacking Islamic State militants showed no immediate sign of increasing strikes in Syria, figures released Wednesday showed, even though the Pentagon chief has said America would intensify its campaign.

Defense Secretary Ashton Carter on Tuesday said the coalition would conduct extra bombing runs against IS jihadists in Iraq and Syria, but only two strikes have been conducted in Syria the last six days.

The diminished tempo comes as Russia finishes the first month of its own Syria bombing campaign, and observers have suggested the US-led coalition is worried about flying close to areas of Russian action.

The Pentagon has repeatedly denied this.

"It has nothing to do with the Russians, nothing whatsoever," coalition spokesman Colonel Steve Warren said. "There has been a reduction in strikes, but it's tied to operations. ... It's really more about the ebb and flow of battle."

As of Wednesday, coalition aircraft had carried out a total of 2,680 air strikes in Syria. The campaign started in June last year, and has also unleashed 7,712 strikes in Iraq.

Pentagon figures show the coalition launched 359 air strikes in July. The number dropped to 206 in August, then just 115 in September.

There have only been 92 this month; the most recent two of these were against relatively low-value targets: an IS vehicle and two mortars.

Unlike Iraq, where the United States has had a presence and local partners for 12 years, Syria remains relatively unchartered in terms of intelligence assets.

Defense officials are working to develop contacts with local partners, and the Pentagon is dropping supplies to a group of about 5,000 fighters in northern Syria in hopes they'll tackle the IS group in its stronghold Raqa -- and provide valuable intel.

"The intelligence in Syria continues to pile up," Warren said, noting that he expected additional Syria strikes soon.

"We are continuing to make ties with forces on the ground in Syria. That will spur target development as well," he added.

Carter said the coalition would support additional ground raids and air strikes, and focus on supporting rebels in Raqa, as well as local Iraqi forces trying to retake the city of Ramadi.

Moscow says its bombing campaign targets IS jihadists and other "terrorists," but the West claims the strikes have focused on moderate rebels fighting Russian-backed President Bashar al-Assad's forces.