Sydney  - Three people are dead, two missing and dozen injured after an explosion on board an asylum-seekers' boat that was being towed to Christmas Island by Australian authorities Thursday.

The boat, which arrived off Australia's west coast from Indonesia, was carrying 49 people believed to be from Afghanistan.

"It's believed an explosion has occurred in the engine compartment of a vessel," Police Sergeant Greg Lambert told Australia's AAP news agency. "It's believed three people have been killed, two people are missing and others are injured. Numbers are not yet known."

The vessel was being escorted by a Royal Australian Navy boat after being intercepted near Ashmore Reef on Wednesday.

The boat and its escort are now on their way to Darwin, the nearest city, where doctors are ready to treat the casualties.

Western Australia's Broome Hospital and Royal Flying Doctor Service have also been placed on alert. The boat is the sixth vessel with asylum seekers on board to arrive in Australian waters this year. (dpa)