U.S.-led coalition

Carter also said that no collaboration is possible as long as Moscow continues to strike other targets.

He said the U.S. will limit its discussions with Russia to basic, technical talks about efforts to ensure that flights over Syria are conducted safely and, "That's it."

Carter also said that no collaboration is possible as long as Moscow continues to strike other targets.

He said the U.S. will limit its discussions with Russia to basic, technical talks about efforts to ensure that flights over Syria are conducted safely and, "That's it."

The news that about 3,000 U.S.-trained Iraqi army soldiers were added to the fight in recent days was disclosed to Defense Secretary Ash Carter, who spent the day getting updates and meeting with U.S. and Iraqi officials and commanders in Baghdad.

It was Carter's first visit to Iraq since he took office in February.

However, Obama conceded more difficulties ahead in fighting what he described as a nimble and opportunistic enemy.

"We're starting to see some progress," the president said during a rare visit to the Pentagon, ticking off a list of towns in Iraq and Syria he said had been wrested from IS control in recent weeks.

Flanked by top military commanders, Obama also warned of the Islamic State's efforts to recruit and inspire vulnerable people in the United States, and called on the American-Muslim community to "step up in terms of pushing back as hard as they can." 

It was one of the largest coalition operations carried out in Syria to date, the coalition said Sunday.

At least 16 airstrikes were reported late Saturday and early Sunday, triggering successive explosions that shook the city and created panic among residents, activists said. 

The U.S.-led coalition often targets IS-held towns and cities in Syria, but the overnight strikes on Raqqa were rare in their intensity.