The US military's top officer Sunday defended the pace of the air war against Islamic State jihadists, warning that escalating bombing raids or sending in more American troops would be a mistake.

During a visit to a French aircraft carrier in the Gulf taking part in the air campaign, General Martin Dempsey appealed for "strategic patience" in the fight against the IS group in Iraq and Syria.

Expanding the air war could risk civilian casualties and play into the hands of IS propaganda, he said aboard the Charles de Gaulle.

"So we have a responsibility to be very precise in the use of air power. And that means that it takes time" to gather accurate intelligence on possible targets, the general said.

"Carpet bombing through Iraq is not the answer."

The tempo of military operations also depended on the strength of the Iraqi army and the Baghdad government's willingness to reconcile with an alienated Sunni population, he said.

The conflict could be decided on the battlefield relatively quickly, but military operations were only part of a broader effort, said Dempsey.

"I do think it's going to require some strategic patience," he said, adding that "these underlying issues have to be resolved".

Dempsey spoke in the carrier's hangar alongside his French counterpart, General Pierre de Villiers, who said he shared the American general's view.

The coalition faced a "paradox" as Western countries wanted "quick results", but the Iraqi army had to be rebuilt before it could take back territory from the IS extremists, de Villiers said.

Despite the pleas for a deliberate approach, Iraq on Sunday urged the international coalition to use its air power to help protect the country's archaeological sites before IS extremists destroyed more precious artefacts.

"We request aerial support," said Iraq's tourism and antiquities minister, Adel Fahad al-Shershab.

Recent attacks on Iraq's historic heritage have taken place in the northern province of Nineveh, where Baghdad lacks troops on the ground.

- Waiting on Iraqi army -

Washington has faced criticism from some Arab allies that the air campaign launched in August is ineffective and overly cautious.

Hawkish US lawmakers have called for sending in more special operations forces to help guide Iraqi troops in combat and direct air strikes.

But Dempsey, chairman of the US Joint Chiefs of Staff, said there was no need to increase the number of American troops advising and training local forces, as the Iraqi army was not ready for a larger-scale effort.

"We've got trainers and advisers that are waiting for some of the Iraqi units to show up," the general said of the 2,600-strong US contingent.

"And when they've shown up, a handful of them, they've shown up under strength and sometimes without the proper equipment."

In a meeting earlier aboard the ship between the top officers and other commanders, Dempsey made similar points, saying military gains would be unsustainable without political progress, a US defence official said.

The four-star US general, who is due to visit Iraq in his regional tour, earlier watched Rafale fighter jets roar off the deck of the carrier, which is carrying out an eight-week mission in support of the war on the IS militants.

Between 10 and 15 warplanes from the De Gaulle conduct combat missions over Iraq every day, French officers said.

In a first for French forces, the De Gaulle is operating in an area under US command, with the American carrier USS Carl Vinson often within sight on the horizon.

The unusual visit by America's top-ranking officer reflected improving ties between the US and French armed forces in recent years, as well as a friendly rapport between the two generals.

"This visit by my friend General Dempsey is symbolic of the quality of the cooperation between our two countries," de Villiers said.

In contrast to tensions over the US invasion of Iraq in 2003, which was vehemently opposed by France, Paris and Washington now mostly see eye-to-eye on the threat posed by the IS extremists.