Image caption The MoD said it has launched an extensive operation to find the soldier

A British soldier has gone missing in southern Afghanistan, the Ministry of Defence has confirmed.

A massive search involving aircraft and ground troops has been launched after he went missing from his base in central Helmand.

He had left the base alone in the early hours of Monday morning, a move which is described as "highly unusual".

The soldier's next of kin have been informed. The Taliban has told the BBC it has killed a soldier in the area.

The BBC's Quentin Sommerville in Kabul said a local Taliban fighter told the BBC that insurgents had captured a foreign soldier in the Babaji area and after a firefight the solider was killed.

'Appropriate action'

But our correspondent said the Taliban often made exaggerated claims for propaganda reasons.

The international mission in Afghanistan, Isaf, denied that any gun battle took place.

Media playback is unsupported on your device Media caption Liam Fox: "All necessary and appropriate action will be taken"

It said it was attempting to find the missing soldier as quickly as possible.

The MoD said in a statement: "A British serviceman is missing in Afghanistan. An extensive operation to locate him is under way."

Prime Minister David Cameron said it was "a very disturbing incident" as he made an unannounced visit to the country.

Defence Secretary Liam Fox said: "The United Kingdom and Isaf are taking all necessary and appropriate action."

Dr Fox also called for restraint by MPs and the media on the case, saying speculation was unhelpful.

A US soldier is believed to be in Taliban captivity in Pakistan.

Sergeant Bowe Bergdahl, 25, from Hailey, Idaho, was captured in June 2009 in Paktika province in eastern Afghanistan.