The United States said Friday that France's announced plans to withdraw its forces from Afghanistan a year early was a "consulted and managed effort" with its allies.

Amid an uproar over the killing last week of unarmed troops, French President Nicolas Sarkozy said after meeting Afghan President Hamid Karzai that France would complete its pullout from the NATO-led mission at the end of 2013.

US State Department spokeswoman Victoria Nuland stressed the importance of a coordinated withdrawal even though US and other NATO countries intend to withdraw their combat forces by 2014.

"We obviously want to continue to work together to ensure that this is implemented in the way that is consistent with the efforts of all of NATO to give increasing authority to the Afghans and that it is smooth," Nuland said.

"But what we are gratified by is that this was not precipitous, that this was worked through carefully with NATO, with the Afghans and in consultation with all of us," she told reporters.

"There were some concerns expressed in NATO countries ... as well as in Afghanistan that whatever was done needed to be done in a consultative fashion, needed to be done in a managed fashion," Nuland said.

"And what we see now is just that, a consulted and managed effort," she said.

"This was a national decision of France. It was done in a managed way. We will all work with it," the spokeswoman said.

Sarkozy made the threat to withdraw French forces ahead of the 2014 deadline for a pullout of all US-led combat troops after an Afghan soldier shot and killed four French soldiers in an attack that left 15 other troops wounded.

After the deaths of the four soldiers, Sarkozy sent his Defence Minister Gerard Longuet to Kabul to evaluate ways to improve the security of the French troops who are training up the Afghan army.

Longuet said he was told the killer was a Taliban infiltrator in the Afghan army, but Afghan security sources said he opened fire because of a video showing US Marines urinating on the dead bodies of Taliban insurgents.

The Taliban, usually quick to claim coalition deaths, said they were investigating and suggested some of the many attacks by Afghan soldiers on their foreign counterparts were prompted by anger towards the "invading enemy".