An Al-Qaeda offshoot this week seized a key underground arms depot in the strategic town of Gao in Mali's rebel-controlled north, security sources said on Sunday.

The Al-Qaeda in the Islamic Maghreb (AQIM) "have taken our underground weapons and ammunition depot in Gao. It's one of the main depots of the Malian army," a security source told AFP.

The depot was built to deal with "the prospect of a long and difficult war," the source said.

A regional security source confirmed the seizure, saying the vast cache of weapons will "really boost AQIM's striking power", and adding: "It is really impressive what AQIM has found in the underground depot."

The source said the group "is today more armed than the combined armies of Mali and Burkina Faso", another west African nation.

Tuareg and Islamist rebels said overnight that they have joined forces to create an Islamic state in the vast desert north.

Mali has been in turmoil since a March 22 coup and the north has steadily slipped out of Bamako's control.

The accord on creating an Islamic state was hammered out between the Islamist group Ansar Dine (Defenders of Faith), backed by AQIM, and the secular Tuareg National Movement for the Liberation of Azawad (Tuareg MNLA).

It comes after weeks of sometimes fraught discussions between two groups that have long held separate objectives and ideologies.

Regional and Western leaders have long feared a breakaway state in Mali's remote desert north could become Al-Qaeda's main safe haven.