France has asked the US to help move its troops to Africa as it pushes an offensive against rebels in Mali, US officials said Monday, adding no final decision on logistical support had been made.

Washington was also working 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.

In a sign of the urgency, Islamist forces seized control of a town in a fresh attack on Mali's government-held south and vowed to strike "at the heart" of France as it waged a fourth day of airstrikes.

Paris had asked for surveillance aircraft, including drones, and aerial refueling tankers, US officials said. France also wanted cargo planes to ferry troops and equipment.

No final decision has been taken, but military officials said details for the logistical support were being worked out.

"They have asked for help in moving French troops to Africa," a defense official said on condition of anonymity.

The US administration would likely approve refueling of French aircraft but "we haven't refueled anything yet," the official told reporters.

State Department spokeswoman Victoria Nuland said the United States was looking at "a number" of requests.

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

"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.

Washington and France were involved in "burden-sharing" in Mali, having divided up the logistics of their support for the interim government, brought in after a March coup.

Under US law, the administration is barred from giving direct aid to Mali until democracy is fully restored in the country.

"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," 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.