By Shihar Aneez

COLOMBO (Reuters) - A court in the Maldives jailed former vice president Ahmed Adeeb for 15 years for plotting to assassinate President Abdulla Yameen in an explosion on the president's launch last September, his lawyer said on Friday.

The judgment was delivered late on Thursday, four days after Adeeb was sentenced to 10 years on terrorism charges for possessing firearms. The trial was held behind closed doors due to security concerns, authorities said.

The secret trial was denounced by the opposition and is likely to raise international concern over the evidence used to convict Adeeb, once seen as a future leader of the Indian Ocean atoll whose popularity as a tourist paradise is at odds with its deeply troubled politics.

"He was sentenced to 15 years based on seven anonymous witnesses. The court said it can conclude that there was an IED (Improvised Explosive Device)," Moosa Siraj, Adeeb's lawyer, told Reuters.

Adeeb's two bodyguards were jailed on Thursday in the same case, each for 10 years. A former chief prosecutor was sentenced to 17 years in jail for conspiring to kidnap Yameen.

"Anyone who is a threat to Yameen's despotism is bound to be found guilty of terrorism charges. Maldives continues to be a travesty of justice," Ahmed Mahloof, spokesman for the Maldives United Opposition, told Reuters via a text message.

The Maldives had asked foreign agencies to examine the evidence of the alleged attempt on Yameen's life that took place just as his launch prepared to dock last Sept. 28.

Notably, the U.S. Federal Bureau of Investigation said it found "no conclusive evidence" of a bomb blast.

"The FBI report clearly said it was not an IED, while Saudi investigators' report said there was no evidence to say it was a bomb. We will definitely appeal against the verdict," said Siraj, the lawyer.

Yameen was unhurt in the blast, which blew off the rear doors of the launch, but his wife and two aides were injured.

Adeeb, 34, was arrested on Oct. 24 after an initial probe. Parliament impeached him on Nov. 5.

Adeeb's sentencing came a week after exiled former leader Mohamed Nasheed, now in exile in Britain, formed a united opposition group aimed at toppling Yameen.

Nasheed was himself jailed for 13 years on terrorism charges after a trial in 2015 that was widely denounced as politically motivated. He was allowed to travel to Britain for medical treatment and was last month granted refugee status by London.

(Editing by Douglas Busvine and Nick Macfie)