Indonesia launched two tsunami alert buoys with US help Wednesday to boost an early warning system for the country worst hit by the 2004 killer wave, the US embassy said.

The devices are part of a 22-buoy system planned for the Indian Ocean, an embassy statement said.

"This work is extremely important for ordinary Indonesians who live and work in tsunami-prone areas," US Ambassador Cameron Hume said in the statement.

The Deep-ocean Assessment and Reporting of Tsunami (DART) system uses a sensor on the seabed to send signals to a buoy on the surface, which in turn transmits the information to authorities on land.

The first buoy was deployed off the Thai resort area of Phuket in December 2005, while Indonesia launched its first in September last year and now has five. A sixth went missing in January when the cable attaching it to the sea floor was severed.

Indonesia and Thailand were among a dozen nations lashed by the catastrophic December 2004 tsunami. More than 168,000 people were killed in Indonesia's Aceh province and Nias island.

Indonesia sits on the so-called Pacific Ring of Fire, where continental plates collide, meaning earthquakes are a regular and often deadly occurrence.

Offshore, shallow quakes can trigger tsunamis, which occur far less frequently, but can hit coastal areas quickly and claim a large number of lives.