Obama vows to keep momentum in fight against IS

US President Barack Obama on Wednesday pledged to keep up momentum in the fight against the Islamic State (IS), saying the 66-member coalition of nations is "on the offensive".

"Today, on the ground in Syria and in Iraq, IS is on the defensive," Obama told reporters after meeting with his national security team at the headquarters of Central Intelligence Agency. "We have momentum, and we intend to keep that momentum."

Though the IS has managed to advance in some areas of Syria and Iraq, it has not had a successful major offensive operation on the ground there since last summer, Obama said.

On Wednesday, the Pentagon said the US-led military operation against the IS has entered its second phase with the aim to dismantle the group, Xinhua news agency reported.

The first phase focused on stopping IS militants from advancing and squashing its ability to operate as a conventional force, said Colonel Steve Warren, the spokesman for the US-led coalition fighting the IS.

At the CIA headquarters, Obama stressed the importance of ending the Syrian civil war, calling it the "only way to truly destroy ISIL (IS)".

"So we continue to work for a diplomatic end to this awful conflict," he said.

The UN special envoy for Syria Staffan de Mistura said Wednesday that the second round of proximity talks aimed at brokering a political end to the Syrian crisis had started.

To date UN-mediated talks have made little progress to end the five-year war which has killed over a quarter million people and displaced millions of others.

On Wednesday, Obama also noted that as the coalition has made it harder for foreign terrorists to reach Syria and Iraq there is an "uptick" in the number of IS fighters heading to Libya.

"We are going to continue to use the full range of tools to roll IS back from Libya while assisting the new and nascent Libyan government as it works to secure their country," Obama said.