The US-led coalition fighting the Islamic State group aims to open a new front in southern Syria in addition to its ongoing offensive in the northeast, US Defense Secretary Ashton Carter said Wednesday.

"We will aggressively pursue opportunities to build pressure on ISIL in Syria from the south, complementing our existing, robust efforts from northeastern Syria," he said in a speech to US troops at Fort Bragg, North Carolina. ISIL is another term for the Islamic State group.

Carter said the move will help Jordanian security and further split IS theaters of operation in Syria and Iraq.

At the moment, the main coalition effort is centered on recapturing the Manbij pocket in northern Syria, the last jihadist-held territory bordering Turkey.

That offensive is being led by the Syrian Democratic Forces, an Arab-Kurdish coalition.

US-backed rebels also have tried to attack IS from the south, but so far without success.

The New Syrian Army, a small rebel formation allied with the United States, tried in June to attack IS positions in Boukamal on the border with Iraq.

But after coming within five kilometers (three miles) of the town, the rebels were repelled by a jihadist counteroffensive.

In another setback, the Pentagon said a rebel camp was bombed in mid-June by Russian warplanes, a charge Moscow has denied.

After two years of war against the jihadists, the coalition estimates it has recovered about 50 percent of the territory seized by IS in Iraq, and 20 percent in Syria.

The allies' objective is to isolate and then recapture the cities of Raqa in Syria and Mosul in Iraq, the most important IS strongholds.

"As this isolation and pressure on Raqa and Mosul builds from the outside in, our partners will continue to reach deep inside those cities to enable pressure on ISIL from the inside out," Carter said.