New Pentagon chief Ashton Carter will hold talks Monday in Kuwait with top US commanders and diplomats to discuss the war effort against the Islamic State jihadist group, officials said.

Carter flew to Kuwait City from Afghanistan on Sunday to chair the extraordinary meeting that will see more than two dozen senior military officers and ambassadors gather at the sprawling US Army base of Camp Arifjan, officials said.

Carter, an experienced Pentagon technocrat who took office last week, "wants it to be an open conversation regardless of rank," a senior US defence official told reporters on condition of anonymity.

The meeting was not intended to produce a new strategy but to allow Carter to better understand the challenge posed by the jihadists and the range of efforts aimed at defeating them, said the official.

Carter will be looking for an update on the military and diplomatic facets of the campaign, including the Iraqi government's efforts to recruit Sunnis into the fight against the IS extremists, the official said.

Commanders believe the air war against the IS and the training of Iraqi army troops is mostly on course, but Baghdad's Shiite-led government has more work to do to persuade Sunnis to take up arms, said the official.

The official said that "on the military side, things are going well" but "it's still yet to be seen how the Iraqis are really going to lead this thing."

"Thousands" of Sunni tribesmen have signed up to fight but a proposed Sunni national guard was still a long way off, the official said.

"Are Sunnis in the fight? Yes. Are they in the numbers we want? No."

- Spreading threat -

Monday's meeting, an unusual gathering of generals, diplomats and intelligence officers, will examine how to counter IS not only in Iraq or Syria but across the region, officials said.

Carter likely will raise questions about "what it means when we have groups swearing allegiance to ISIL in Libya, in Egypt, in Afghanistan," said the official, using another acronym for IS, and "how are we thinking about the next few years of the counter-terrorism fight."

The talks follow more than six months of US-led air strikes that have halted IS advances for the most part and enabled Kurdish forces to recapture some ground in northern Iraq and the Syrian border town of Kobane.

But the jihadists still hold large swathes of territory seized last year across Syria and Iraq and appear to have spread their influence to Libya.

In recent days, IS has claimed responsibility for bombings in Libya as well as the murder of 21 Coptic Christians, most of them Egyptian.

The meeting will include the commander running the anti-IS campaign, Lieutenant General James Terry, as well as the heads of US Central Command, Africa Command, European command, Special Operations Command and Joint Special Operations Command, said Pentagon spokesman Rear Admiral John Kirby.

Diplomats and civilian officials due to take part include John Allen, President Barack Obama's envoy to the anti-IS coalition as well as the US special envoy to Syria, Daniel Rubinstein, he said.

US ambassadors to Egypt, Jordan, Kuwait, Saudi Arabia and the United Arab Emirates will also attend.

Senior intelligence officials also will be on hand for talks that are meant to transcend bureaucratic divisions and information "stovepipes," officials said.

Before landing in Kuwait Carter told reporters he had called the meeting because he was "trying to assess the situation in Iraq, Syria, and the region more generally".

The IS threat was "a regional issue", Carter said, "and I wanted to have all of that expertise represented".

On his first trip abroad as defence secretary since being sworn in last Tuesday, Carter came to Kuwait after a two-day visit to Afghanistan, consulting commanders about the pace of a US troop withdrawal.

Carter will not be travelling to Iraq on the trip, but he said he will head to the country "soon" to meet with some of the more than 2,600 US troops deployed there.