The United States said Friday it shared France's goal of denying haven to terror groups in Mali, after Paris used air power to back a government assault on Islamist rebels.

"We are monitoring the situation closely," National Security Council spokesman Tommy Vietor told AFP.

"We have noted that the government of Mali has asked for support, and we share the French goal of denying terrorists a safe haven in the region."

State Department spokesman Victoria Nuland meanwhile said that Washington was "consulting very closely" with the French government on the fighting.

France's President Francois Hollande announced earlier that French forces were actively supporting an offensive by Malian government troops against the Islamist rebels who control the north of the country and are moving south.

"The operation will last for as long as is necessary," Hollande said in a brief statement, giving no details of the scale of French support.

Sources said Hollande had decided to authorize the deployment of French forces early Friday in agreement with Mali's interim president Dioncounda Traore, after the rebels said they planned to move on the capital Bamako.

Washington has not been asked by the Malian government for any help dealing with the current crisis, Nuland said.

"Obviously we remain deeply concerned by the recent events in Mali. We echo the international community's condemnation of these recent aggressive acts."

But she refused to clarify whether the US stood ready to send in American forces. "I'm not going to get into hypotheticals about what France might need or requests that haven't yet come to us," she said.

US military officials said that American forces had no role in the French intervention at this point. But US military officers were closely monitoring the "evolving" situation.

French Defense Minister Jean-Yves Le Drian said at a press conference that he had spoken with his American counterpart Leon Panetta about the situation.