JAKARTA: A strong 7.5-magnitude quake struck Indonesia's Sumatra island on Wednesday, sparking a local tsunami alert and triggering panic, US meteorologists and residents said.

The US Geological Survey said the undersea quake struck at 3:08 pm (0808 GMT) some 312 kilometres (194 miles) west-southwest of the North Sumatra capital Medan, at a depth of 34 kilometres.

Indonesia's meteorological agency, which put the magnitude of the quake at 6.6, said it struck 42 kilometres northwest of the town of Sinabang, the main town on Simeulue island, in Aceh province.

A 5.5-magnitude aftershock hit shortly afterwards, it said.

The Hawaii-based Pacific Tsunami Warning Centre issued a local watch bulletin, saying there was no threat of a "destructive widespread tsunami" but that a local tsunami could affect coastal areas within a 100-kilometre radius.

Zainul Tahar, the head of Aceh's search and rescue office, told AFP that he had spoken to officials at the port authority in Sinabang.

"I was told that there was no damage, although there was mass panic in town," he said, adding that he had also contacted authorities in Meulaboh, opposite the island on Aceh's mainland, and no damage was reported there.

The meteorology office's Suharjono told ElShinta radio that there were no immediate reports of damage.

However, he said, "the possibility of damage is there... in Sinabang, the possibility of damage exists."

An AFP correspondent in the Aceh capital of Banda Aceh said the quake was felt there for about two minutes.

People initially ran outside in panic, but quickly calmed down, the reporter said.

ElShinta radio also quoted listeners as saying panic had hit the population in North Sumatra's capital of Medan, where people rushed out of buildings.

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

The earthquake-triggered Asian tsunami in December 2004 killed some 168,000 people in Aceh, which is located at the northern tip of Sumatra. Indonesia was the nation worst hit by the tsunami.

Simeulue was one of the islands closest to the 2004 quake's epicentre, but the tsunami killed fewer than 10 people in part because the 70,000-strong population recognised the receding sea as a sign of disaster and fled inland.

In 2005, entire villages on Simeulue were destroyed by a quake which killed at least 17 people. - AFP/ac