Government troops advancing into Tamil rebel-held areas in northern Sri Lanka have found a "submarine" vehicle under construction by the rebels to be used in their fighting against the Navy, military officials said Thursday.

It was detected in a boat factory as troops advanced into the area of Udayarkattukulam 370 kilometres north east of Colombo.

The operation also unearthed three more smaller underwater vehicles still under construction, three boats which could be used for a suicide attack, and a Dvora type fast-attack craft.

The largest underwater vehicle was about 35 feet long and armour- plated, officials said.

"This was the most startling recovery made by troops so far during the ongoing counter-terrorist operations against the LTTE", the defence ministry said.

"With this discovery the LTTE will go down in the history as the first terrorist organization to develop underwater weapons", the ministry added.

It posted pictures of the vessel on its official website (defence. lk). Earlier, advancing troops found a bomb- and mine- manufacturing facility.