BISSAU (Reuters) - Guinea-Bissau's interim leader returned home on Sunday after weeks abroad seeking medical treatment and pledged to organize elections in his coup-prone nation before the end of the year.

President Manuel Serifo Nhamadjo refused to comment on this month's U.S. drug sting that targeted his country's top brass, accused of trafficking Latin American cocaine, but said he expected leaders to unite so donors could fund the election.

African and Western diplomats are pinning their hopes on the election drawing a line under decades of instability in the former Portuguese colony, but a U.S. sting operation that targeted the military chief sent shockwaves through the tiny nation.

"Presidential elections will take place this year," Nhamadjo told reporters after he returned from weeks of treatment in Germany for an unspecified medical problem.

"The political parties will come together to set up, as soon as possible, a unity government," he added.

The nation was thrust into its latest crisis last year when the military arrested then Prime Minister Carlos Gomes Junior and acting President Raimundo Pereira in the midst of an election that Gomes Junior was poised to win.

Armed Forces Chief General Antonio Indjai was briefly in power before officially ceding to Nhamadjo.

However, Indjai is still widely seen as the nation's most powerful man and was targeted by, but escaped, the U.S. sting operation that netted the country's former navy chief.

Nhamadjo refused to comment on the sting operation, which has led to authorities during his absence accusing Washington of illegally kidnapping one of the country's citizens.

"I was away from the country ... I was never briefed on this issue while I was away," he said.

Elections were due to be held in May, but in March West African leaders prolonged the mandate of the caretaker government until the end of the year.

(Reporting by Alberto Dabo; Writing by David Lewis; Editing by Jon Hemming)