Former Maldives deputy leader Ahmed Adeeb has been convicted of plotting to assassinate the president, and sentenced to 15 years in prison - four days after being sent to jail for 10 years in another case.

Following a closed-door trial on Thursday, the Criminal Court found ex-vice president Adeeb and two of his military bodyguards guilty of attempting to kill President Yameen Abdul Gayoom, by setting off a bomb on his speedboat in September 2015.

Gayoom escaped unhurt, but his wife and two others were slightly injured.

After being enlisted to look into the incident, FBI investigators reported there was no evidence that a bomb was behind the blast.

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Yet, Adeeb was arrested by the police and tried in secret following allegations he wanted to topple Gayoom.

With the sentencing, almost all of Gayoom's key rivals are either living in exile or in jail in the island nation, which has been rocked by political turmoil in recent years.

Adeeb's lawyer, Moosa Siraj, told the Maldives Independent website that the court warned him not to criticise the trial.

"The Criminal Court has barred me from calling the trial unfair, but we have concerns and intend to launch an appeal immediately," Siraj said.

Another lawyer, who declined to be named, said Adeeb's two bodyguards who were also convicted on Thursday were sentenced to 10 years each.

Political turmoil

Reporters were barred from the trial after the court invoked national security concerns and said it would not make the hearings or verdict public.

Adeeb, 34, once a close confidant of Gayoom, had enjoyed a meteoric rise until he was dramatically impeached in November after being accused of the assassination plot.

On Sunday, Adeeb was jailed for 10 years on an unrelated "terrorism" charge relating to his role in cracking down on an anti-government protest in May last year.

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President Gayoom is accused of jailing and silencing dissidents in the country, a popular holiday destination for honeymooners.

The latest developments come weeks after ex-leader Mohamed Nasheed, the country's first democratically elected president, was granted asylum in Britain.

Nasheed was sentenced to 13 years in prison on controversial "terrorism" charges last year, but was allowed to travel to the UK for surgery in January and was granted political asylum last month.

About four months ago, the president also secured the jailing of Sheikh Imran Abdulla, leader of the opposition Islamist Adhaalath Party, for 12 years.

Source: Agencies