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JAKARTA, May 24 (Reuters) - An strong undersea earthquake on Thursday off central Indonesia's Nusa Tenggara island chain sparked panic and prompted a brief tsunami warning, but there were no reports of casualties or major damage.

The quake struck near Sumbawa island at a depth of 19 km (11 miles), Fauzi, head of seismology at Indonesia's meteorological and geophysics agency, told Reuters.

"We have cancelled the warning. The quake had no tsunami potential," Sri Woro, the head of the agency, told Reuters.

Residents in Raba town on Sumbawa island said the quake sparked panic but no damage was reported.

"It happened for a few seconds. People were rushing out of their houses and buildings," Eli, who works at the Raba regent's office, told Reuters by telephone.

A local hospital official, Suharto, said some hospital walls in Raba were cracked.

The United States Geological Survey said the quake was of magnitude 5.5 and at a depth of 50.4 km (31 miles) and was 1,380 km (860 miles) east-southeast of the Indonesian capital, Jakarta.

Thailand's Disaster Warning Centre said the quake measured 5.6 there and no tsunami warning had been issued, while Malaysia's seismology agency reported much the same.

Earthquakes are frequent in Indonesia, the world's fourth most populous country. Its 17,000 islands sprawl along a belt of intense volcanic and seismic activity, part of what is called the "Pacific Ring of Fire".

A devastating Indian Ocean tsunami triggered by a giant undersea earthquake off Indonesia's Aceh province in December 2004 left hundreds of thousands dead or homeless across the region.

Nearly 1,600 people were killed in Nusa Tenggara islands when a strong quake triggered a tsunami in December 1992. (Additional reporting by Enny Nuraheni, Harry Suhartono, Adhityani Arga, and Mita Valina Liem)

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