The US-led military operation against the Islamic State (IS) extremist group in Iraq and Syria has entered the second phase with the aim to dismantle the group, the Pentagon said Wednesday.

"Phase one of the military campaign is complete, and we are now in phase two, which is to dismantle this enemy," said Colonel Steve Warren, the spokesman for the US-led coalition fighting the IS.

The first phase focused on stopping the IS militants from advancing and squashing its ability to operate as a conventional force, Warren said.

"We've struck leaders, supply lines, fighters, industrial base and funding sources in both Iraq and Syria," said Warren, claiming that the IS has been weakened and thus lost ability to take hold of any key terrain for almost a year now.

He admitted that fighting is intense now in the Tigris River Valley, which is crucial for the Iraqi Security Forces to recapture Mosul under the help of the US-led coalition.

There is a concern that the battlefield advance achieved by the West-backed forces in Iraq and Syria has so far done little to stop the expansion of the IS militants to Europe, North Africa and Afghanistan. Military experts here have repeatedly warned the fight against the IS militants is likely to drag on for years.