Defence chiefs from the United States, France, Britain and four other nations meet in Paris on Wednesday to examine ways to accelerate gains against Islamic State, including by potentially ramping up the number of police and army trainers.

US Defence Secretary Ash Carter described the gathering as a chance for face-to-face talks among the core contributors in the US-led coalition, which also includes Germany, Italy, Australia and the Netherlands.

"I'll be soliciting their views and describing to them my thoughts about how we can accelerate the campaign, including the variety of capabilities, military capabilities, that will be required," Carter said, predicting increases in the numbers of trainers in the months ahead, including of police who can help hold territory seized from Islamic State.

France was the first country to join US-led air strikes in Iraq. Since the Paris attacks by Islamic State militants in November, President Francois Hollande has stepped up French aerial operations against Islamic State, including in Syria, contributing about 20% of coalition strikes.

A French defence ministry official, speaking on condition of anonymity, said the coalition would discuss ways to broadly intensify the effort.

"It's not just about adding more planes, but also trainers to accelerate the speed with which local forces can retake territory against Daesh," the official said, using a colloquial term to describe Islamic State.

NO ARAB NATIONS AT MEETING

In a telling sign, no Arab states from the region are joining the gathering of top contributors to the campaign. A senior US defence official acknowledged that many Arab allies have been occupied with the Saudi Arabian-led campaign against Houthi militants in Yemen.

Carter said he would be discussing with his allies how to draw a greater contribution from the Sunni Arabs, many of whom view the US-backed, Shiite-led government in Baghdad with suspicion. They also accuse the United States of not moving firmly enough against Syrian President Bashar al-Assad.

"That's something I want to hear from my counterparts over the next couple of days. How can we get them in the game. I have long said that Arabs, and Sunni Arabs, need to get in the game," he said.

Officials said a key focus would be finding ways to increase contributions from other nations outside the "core contributors" to the coalition effort. That could include trainers.

Republicans have sought to portray Obama's strategy to defeat Islamic State as flawed and insufficient, as the militants plot or inspire attacks far beyond their self-declared caliphate in Iraq and Syria.

But last week, Carter offered US troops heading to Iraq an upbeat assessment that emphasized advances by Iraqi forces -- including retaking control of the city of Ramadi -- and by US-backed rebels in Syria.

He focused on efforts to "collapse" the Islamic State's power centres of al Raqqa, in Syria, and Mosul, in Iraq.

The French official said that while there was pressure to intensify the air effort, the US-led coalition's ability to ramp up strikes was limited by "extremely demanding" requirements to avoid civilian casualties.

"It's not about bombing haphazardly, but to give a military impulse to the disorganisation of Deash's command structure and to strip it of economic resources," the official said.