BISSAU (Reuters) - Soldiers briefly detained Guinea Bissau Prime Minister Carlos Gomes Junior, freed a suspected coup leader and said they had ousted the army chief of staff on Thursday in what one diplomat said appeared to be a coup attempt against Gomes.

Former navy chief Bubo Na Tchuto, suspected of leading a 2008 coup attempt in the tiny West African state, had returned from exile in December and immediately sought shelter on U.N. premises in the capital Bissau.

Na Tchuto still holds sway with parts of the armed forces and his return increased tension in a fragile state that is a hub of drugs trafficking to Europe. The United Nations agreed in January to hand over him over to Gomes's government but the transfer did not take place.

"(Gomes) was detained this morning. Bubo Na Tchuto has voluntarily left the U.N. compound. The events are related," a Western diplomat in Bissau told Reuters by telephone.

"It looks like a coup d'etat against the prime minister and the chief of staff."

Soldiers apparently loyal to Na Tchuto said they had replaced the chief of staff, Admiral Jose Zamora Induta, with his deputy, General Antonio Njai.

Gomes's press attache Mamadou Diao subsequently confirmed Gomes had been released and a Reuters witness saw his vehicle heading towards the office of President Malam Bacai Sanha.

The Reuters witness said Na Tchuto had driven off with soldiers to an undisclosed location. The capital Bissau was calm, with some banks and shops shutting and little traffic in the streets.

Na Tchuto took refuge in the U.N. offices in December after returning from exile in nearby Gambia in a canoe, disguised as a fisherman.

He was wanted in connection with a failed 2008 coup attempt against then-president Joao Bernardo Vieira. Vieira was killed by renegade soldiers in March 2009 and replaced by an elected government.