MOGADISHU (AFP) - Heavily armed militants launched a major bomb attack and armed assault against Somalia's heavily-fortified presidential palace late Tuesday, police and witnesses said.

The attack bore all the hallmarks of Somalia's Al-Qaeda-linked Shebab rebels, who have launched a string of strikes against the country's internationally-backed but fragile government.

"There was an attack on the presidential palace, with gunmen attacking a checkpoint at the rear of the compound," police officer Ali Hussein told AFP.

"There was a major explosion and security forces are fighting them," he added, without confirming if the insurgents had managed to enter the city centre compound, known as the Villa Somalia.

Witnesses said there was heavy gunfire and several heavy blasts believed to be grenades.

"Bullets are flying around coming from the palace," said Halimo Nure, who lives close to the compound, one of the most heavily defended areas of the city. "There is shooting and gunfire, there are also explosions like they are using grenades."

The attack appeared to be a repeat of a Shebab assault against the presidential palace in February.

In May the Islamist insurgents also launched a similar attack against the national parliament.