A moderately strong 5.9-magnitude earthquake hit Indonesia's Maluku Sea on Monday, the US Geological Survey said, but no tsunami alert was issued.

The quake struck at a depth of 36.5 kilometers, 85 kilometers northwest of Kota Ternate at 4.30am (1930 GMT Sunday), the USGS said.

The Pacific Tsunami Warning Center said there was no tsunami threat and there were no immediate reports of damage.

Indonesia sits on the Pacific "Ring of Fire" where continental plates collide, causing frequent seismic and volcanic activity.

A huge undersea quake in 2004 triggered a tsunami that killed more than 170,000 people in Aceh province, on western Sumatra island, and tens of thousands more in other countries with coasts on the Indian Ocean.