MALE (Reuters) - The president of the Maldives was unhurt but his wife and two aides were injured in an explosion on board their speedboat as it approached Male, capital of the Indian Ocean archipelago, a senior aide told Reuters.

The cause of Monday's blast was not immediately clear, but it was witnessed by several journalists who were waiting to meet President Abdulla Yameen on his return from the Haj pilgrimage to Saudi Arabia.

"There was a explosion at President Yameen's official speedboat near Male jetty. The president is unhurt, but the first lady was slightly injured," Mohamed Hussain Shareef, minister at the president's office, told Reuters.

Yameen, 59, is a controversial figure both at home and abroad. He rose to power in a bitterly contested presidential election in 2013.

The leader he defeated, Mohamed Nasheed, was jailed earlier this year for terrorism in a case that has drawn international criticism. Human rights lawyer Amal Clooney, wife of Hollywood actor George Clooney, called for international sanctions against the Maldives after visiting Nasheed this month.

A broader crackdown against political dissent belies the reputation of the Maldives as a holiday paradise, with radicalized youths enlisting in significant numbers to fight for Islamic State militants in the Middle East.

Yameen had landed earlier at the main international airport of the Maldives, an island chain with a population of 400,000, and was making the short final journey to Male by water with his entourage.

"We don't know if the explosion was from explosives or the inboard engine," said Shareef, reached by telephone from Sri Lanka.

A Reuters eyewitness saw Yameen walking and helping an injured victim soon after the explosion, which occurred when his speedboat was just six feet (two meters) from the main jetty.

Shareef said Yameen had accompanied his wife, Fathimath Ibrahim, to Male's main hospital. A bodyguard who suffered serious injuries and a presidential aide who was slightly hurt were also taken to hospital.