The US military secretly deployed a small number of trainers and advisers to Somalia in October, the first time regular troops have been stationed in the war-ravaged country since1993, when two helicopters were shot down and 18 Americans killed in the "Black Hawk Down" disaster.

A cell of US military personnel has been in the Somali capital of Mogadishu to advise and coordinate operations with African troops fighting to wrest control of the country from the al-Shabab militia, an Islamist group whose leaders have professed loyalty to al-Qaeda.

The previously undisclosed deployment - of fewer than two-dozen troops - reverses two decades of US policy that effectively prohibited military "boots on the ground" in Somalia. Even as Somali pirates and terrorists emerged as the top security threat in the region, successive presidential administrations and the Pentagon shied away from sending troops there for fear of a repeat of the Black Hawk Down debacle.

In recent years, the Obama administration has slowly and cautiously become more directly involved in Somalia.

Drones from a US base in Djibouti - a neighbouring Horn of Africa country - conduct surveillance missions and occasional airstrikes from Somalia's skies. Elite Special Operations forces have also set foot on Somali territory on rare occasions to carry out counter-terrorism raids and hostage rescues, but only in the shadows and for no more than a few hours at a time.