The United States said Monday it was ready to share intelligence and provide logistical support to French forces fighting militants in Mali.

US Defense Secretary Leon Panetta pledged assistance to the French intervention in Mali and said Al-Qaeda and its allies would be denied safe haven anywhere in the world.

"I commend France for taking the steps that it has," he told reporters aboard his plane bound for Lisbon.

"What we have promised them is that we will work with them to cooperate with them and to provide whatever assistance we can to try to help them in that effort," said Panetta, according to a Pentagon transcript.

The defense chief said the assistance would take the form of logistical support, including aerial refueling tankers, and intelligence sharing.

The aim was to reverse the militants' momentum and safeguard cities, though Panetta could not predict how long the military intervention would last.

"It's hard to estimate the timeframe here because obviously, the effort is to try to do what is necessary to halt their advances and to try to secure some of the key cities in Mali," he said.

Panetta said decisive action was needed to deny Al-Qaeda in the Islamic Maghreb (AQIM) a sanctuary in Mali that could be used to stage attacks on the West.

"We have a responsibility to go after Al-Qaeda wherever they are... And we have a responsibility to make sure that Al-Qaeda does not establish a base for operations in North Africa and Mali."

As the US military prepared to back up the French effort, militant fighters in Mali fled bases after a fourth day of French airstrikes but seized a small town in the government-held south and vowed retaliation against France.

Defense officials in Washington said the United States was not planning to deploy troops or conduct bombing raids but acknowledged that recent advances by forces aligned with Al-Qaeda had raised concerns about a growing extremist threat.

US commanders were looking at providing cargo planes to help transport French troops, tankers for refueling French fighters and manned and unmanned surveillance aircraft to gather more detailed intelligence, the officials said.

However, no formal decision had been taken partly because a coup last year in Mali has created a legal dilemma, a defense official said on condition of anonymity.

The US administration is barred by law from giving direct aid to Mali until democracy is fully restored.

"We are, in concert with the security track, pushing hard on all stakeholders in Mali to commit to and begin preparing for the elections that are supposed to take place by April of this year," State Department spokeswoman Victoria Nuland said.

She added that both Paris and Washington shared the goal of restoring democracy in Mali, and insisted the United States had no difficulty letting France take the lead on the ground.

"It speaks to the strength of our allies and our ability to share (the) burden around the world with them. It's a good thing," the spokeswoman said.

Washington was also collaborating with ECOWAS to speed up plans for an African-led peacekeeping force in Mali, where the interim government is working with French backing to stop rebels marching on the capital, Bamako.

Nuland said Washington was working with the 15-nation Economic Community of West African States "to encourage them to accelerate the deployment of their troops," she said.

"There are a number of African countries who are starting to express a willingness to go."

The US was also prepared to use its Africa contingency operations training unit to fly American training staff "this week" out to countries that might be ready to deploy, Nuland said.