We have no knowledge why the plane diverted... but I can say that it was not to be done

Air Marshal Jayalath Weerakkodi

The incident is the latest to test a ceasefire signed in February and comes as the two sides prepare for landmark peace talks due in June.

The Tigers' chief negotiator, Anton Balasingham, flew out of northern rebel-held territory on Wednesday, bound for the Maldives, the authorities had been told.

But the five-seat plane suddenly changed course and flew the other way, over Trincomalee, an area which has seen many battles in the 20-year civil war.

The Island newspaper said Mr Balasingham's plane had nearly been shot down because Air Force Eastern Command had been taken unawares.

A major crisis was only averted after Air Force High Command in Colombo intervened.

Air Marshal Jayalath Weerakkodi said on Monday he still did know why the plane had diverted.

"But I can say that it was not to be done," he said on Maharaja television.

A complaint had been lodged with the peace monitoring group set up by Norway, which brokered the truce and is mediating talks, he added.

Tension

Mr Balasingham, who is based in London, had spent a month in the rebel-held north meeting Tiger commanders ahead of the June talks.

Mr Balasingham had agreed to fly to the Maldives

As he left, the navy said it had intercepted three boats carrying 40 rebels near Trincomalee.

No fighting took place. Peace monitors were called in and the rebels were then allowed to disembark.

Defence Secretary Austin Fernando told the BBC the government would try to make sure there were peace monitors stationed on board all navy ships in the future.

He said in future such incidents could be avoided if the Tigers informed the monitors of their movements.

More than 64,000 people have died in Sri Lanka's civil war since rebels began fighting for independence for the island's minority Tamils in 1983.