JAKARTA, Dec 4 (Reuters) - An earthquake measuring 6.0 on the Richter scale shook the northern part of Indonesia's Sulawesi island on Tuesday, but there were no immediate reports of damage or casualties, the meteorology agency said.

The undersea quake was at a depth of 35 km (22 miles) and was centred 138 km northeast of Bitung on the northern tip of Sulawesi island.

"We are still looking for damage and casualties from civilians. It was felt in Bitung, Manado, and Ternate," said Suharjono, an official at the meteorology agency.

Indonesia, which is situated on a belt of intense seismic activity known as the "Pacific Ring of Fire", has had a series of earthquakes in recent weeks, but experts say that this activity is normal.

A huge earthquake in December 2004 triggered a devastating tsunami in the Indian Ocean which killed more than 230,000 people in the region, including 170,000 Indonesians. (Reporting by Harry Suhartono and Mita Valina Liem; Editing by Sanjeev Miglani)

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