The Somali terrorist outfit al-Shabaab has threatened to strike at American targets around the world days after it mounted a deadly and unprecedented attack on US forces stationed in Kenya.

Emboldened by the unexpected success of last Sunday’s assault on a military base in northern Kenya’s Manda Bay region, the jihadist group, a franchise of al-Qaeda, promised “further bloodshed” against American soldiers and civilians alike.

The attack, which exposed a troubling lack of security, leaves the Trump administration struggling to deal with a significant and growing military challenge at a time when it is already preoccupied by its face-off with Iran.

The US Army’s Africa Command (AFRICOM) has scrambled to respond to the attack, sending combat-ready reinforcements from the 101st Airborne Division to northern Kenya and stepping up airstrikes against al-Shabaab in neighbouring Somalia.

But it has yet to explain how a lightly-armed detachment of Islamist fighters, probably numbering no more than 15, managed to cross into Kenya, advance unspotted on foot across 50 miles of bush before killing three Americans and wrecking six hi-tech surveillance aircraft.

Although al-Shabaab has waged an insurgency against the Somali government and foreign forces stationed in Somalia for more than a decade, it was only in September that the group began to target the United States in earnest.