At least six people have been killed in a firefight after gunmen attacked an army barracks in Guinea-Bissau, military sources say. They say the army repelled the pre-dawn attack just outside the capital Bissau, killing six "rebels". The raid is likely to further heighten tensions in the West African nation, where the military seized power in a coup in April. Instability has turned the country into a prime drug smuggling spot. Gangs use the country to smuggle cocaine from Latin America to Europe, allegedly in collusion with top army officers. The UN Security Council has recently demanded that Guinea-Bissau's rulers restore constitutional rule. Guinea-Bissau has a long history of coups since independence from Portugal in 1974, and no elected leader has finished their time in office since.