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.