An Al-Qaeda offshoot has seized a key arms depot in Mali, security sources said Sunday, deepening the crisis facing the country after rebels declared an independent Islamic state in the north. The underground weapons and ammunition depot in the strategic northeastern town of Gao will vastly boost the firepower of Al-Qaeda in the Islamic Maghreb (AQIM), sources said, even as the coup-torn country's embattled transitional government struggled to assert its authority over the vast desert north. "It's one of the main depots of the Malian army," a security source told AFP, adding that it had been built in case of "a long and difficult war." 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", Mali's neighbour to the east. News of the seizure, which happened earlier this week, emerged Sunday after the interim government formed following a March 22 coup rejected the declaration of a separate Islamic state in Mali's vast desert north. The declaration was made by Islamist movement Ansar Dine (Defenders of Faith), backed by AQIM, and secular Tuareg group the National Movement for the Liberation of Azawad (MNLA). "The government of Mali categorically rejects the idea of the creation of an Azawad state, even more so of an Islamic state," Hamadoun Toure, information minister in the transitional administration, told AFP. "Even though this state creation is just on paper and not de facto, we are coming forward to stress that Mali is secular and will remain secular," he said. Transitional leaders have stressed their wish to restore the country's territorial integrity but seem unable to guarantee their own safety, let alone mount a credible challenge against the north's new masters. Toure's remarks came as interim president Dioncounda Traore was in Paris for medical treatment after being assaulted by protesters who stormed his office last week -- underlining the turmoil gripping a nation once seen as an example of democracy in the region. His entourage said tests had revealed nothing alarming and that Traore was expected back in Mali in the coming week. The accord between the MNLA and Ansar Dine follows weeks of sometimes fraught discussions between two movements, which have long held separate objectives and ideologies. Ethnic Tuareg rebels, many of whom were mercenaries for Moamer Kadhafi in Libya and returned heavily armed to their homeland, rekindled their decades-old struggle for autonomy with a massive offensive in mid-January. A coup by Captain Amadou Sanogo and a group of low-ranking officers ousted the government in Bamako, saying it was incompetent in handling the Tuareg rebellion. But the coup only opened the way for the Tuaregs, Ansar Dine -- led by the charismatic Ag Ghaly and backed by AQIM -- and criminal groups to occupy the vast north of the country, an area larger than France. The agreement between the Tuareg MNLA and Ansar Dine leaves AQIM's position in "Azawad" unclear but creates a fresh headache for the transitional authorities in Bamako and the Economic Community of West African States (ECOWAS). Regional and Western leaders have long feared a breakaway state in Mali's remote desert north could become Al-Qaeda's main safe haven. ECOWAS, which is playing the role of mediator in the Mali crisis, reacted ambivalently to the declaration by Ansar Dine and the MNLA. "It's always better to negotiate with one single group than with several groups whose interests are sometimes diametrically opposed," said Burkina Faso Foreign Minister Djibrill Bassole, whose country is leading the talks. But he said mediators reject any solution that splits the country in two. "The essential thing (is) that the group choose the option of a negotiated solution to the conflict," he added, calling on the newly formed alliance to "abandon terror and terrorism". In Gao, the sealing of the rebels' deal was greeted by the sound of guns being fired into the air, local residents said. "Allah has triumphed," declared Sanda Ould Boumama, an Ansar Dine spokesman in the northern city of Timbuktu.