The United States on Wednesday described the new prime minister of Mali as a man respected by US officials and urged him to move swiftly to form a new government to lead the troubled country.

Diango Cissoko was appointed prime minister by the west African nation's interim president, Dioncounda Traore, just hours after Cheick Modibo Diarra quit as premier under pressure Tuesday from former putchists.

"Django Cissoko... is somebody that we know and we respect, and we want to see him now form a government that can provide political leadership going towards an election in the spring, or as soon as we can get there," State Department spokeswoman Victoria Nuland said.

But she reiterated that Washington continued to have concerns about what former coup leader Captain Amadou Sanogo "is up to, and we think that he and his forces need to be marginalized so that we can have real security both in Bamako and moving north."

Interim authorities in Bamako have remained deadlocked and powerless in the wake of the occupation of northern and eastyerm Mali by Al-Qaeda linked extremists who piggybacked on a Tuareg rebellion that kicked off in January.

A March coup by frustrated soldiers did not fail to stop the rebel advance, and Islamists seized the vast but mostly arid north in a matter of days, later chasing out their Tuareg allies.

The United States and allies have been hammering out plans for a UN-mandated African-led force to be deployed in northern Mali to root out Islamist militias which have unleashed a reign of terror on the region.

But differences have flared over how and when to deploy the force, with US envoy to the United Nations Susan Rice reportedly clashing sharply with France over a plan put forward by the African Union.

Nuland stressed Washington wanted "a draft that is going to hold up" and produce results, adding "we're very much in consultations with our partners. We're working it through with the French now, as allies do."

One of differences appeared to be over whether to deploy troops straight away in northern Mali, or whether it should happen in two stages to allow some training of the forces first.

"We want to ensure that once we get to UN Security Council action that we're going to end up with a force that can do what it's mandated to do, can really provide security on the ground, can meet the needs, can help Malian forces go north," Nuland said. "So these things have to be worked through carefully."

Rice told reporters at the United Nations the United States wanted "the swiftest possible restoration of democratic government in Bamako" and was committed to ensuring there was "not an enduring safe haven for terrorists in the north of Mali."

She said the UN Security Council was working on a resolution to "address in particular the challenge of restoring sovereignty and territorial integrity and ridding terrorists from the north of Mali."