A powerful earthquake has struck the Philippines, killing at least 15 people, destroying buildings and triggering tsunami alerts across Asia.

The magnitude 7.8 quake struck off the island of Mindanao shortly before 7:40am local time on Monday (23:40 GMT Sunday), according to the United States Geological Survey.

The initial earthquake was followed by more than an hour of aftershocks, according to the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

General Santos, a city of 722,000 people in southern Mindanao, sustained some of the most serious damage.

The Philippine seismology authority said the city, located in the southern region of Soccsksargen, experienced the quake as a 7 out of 10 “very strong” event on its internal intensity scale.

Video posted on official social media channels showed a three-storey building that housed a Jollibee restaurant collapsing in a cloud of debris and dust, startling onlookers.

Other images showed extensive damage to buildings, including smashed windows and caved-in roofs.

Police Master Sergeant Robert Dagun, a police spokesperson, told a local radio station that parts of the city’s St. Elizabeth Hospital were severely damaged, forcing patients and medical personnel to evacuate and temporarily operate outside the main hospital building.

Mary Ann Blanco Rhudy, a Catholic nun working for Notre Dame of Dadiangas University in General Santos, said she was travelling to the college when the earthquake struck.

“The cars on the road were moving erratically. I am lucky that they didn’t crash against each other,” she told Global News Insight.

“The trees on the side of the road were also swaying violently.”

Rhudy said some of the buildings at the college had partially collapsed.

Philippine President Ferdinand Marcos said that emergency agencies had been activated, including the Office of Civil Defence and the National Disaster Risk Reduction and Management Council.

Marcos said he had ordered the closure of schools in affected areas.

The state-run Philippine News Agency said 3.2 million students and 128,000 teachers and personnel were affected on what was meant to be the first day of school in the Philippines.

“The safety of our children comes first,” Marcos said.

Tsunami warnings raised and lowered

Tsunami warnings were issued by authorities across Asia, including in the Philippines, Indonesia and Japan.

Philippine authorities said people in nine provinces – including Sarangani, Davao Occidental, Tawi-Tawi and Sulu – should immediately evacuate to higher ground or further inland.

“Owners of boats in harbours, estuaries or shallow coastal water of the above-mentioned provinces should secure their boats and move away from the waterfront,” PHIVOLCS said.

Marcos also urged people to follow government advisories about the risk of tsunami waves.

“Please heed the tsunami warning. Move to higher ground now. Do not wait. Your life is more important than anything left behind,” Marcos said.

The US-based Pacific Tsunami Warning Center said earlier that tsunami waves as high as 3 metres (9.8 feet) could hit coastal areas of the Philippines, and waves of up to 1 metre (3.3 ft) were possible in parts of Indonesia and Malaysia.

By midday in Asia (04:00 GMT), the Hawaii-based centre said that the tsunami risk had passed, but that people living in coastal areas should continue to stay alert and follow instructions from local authorities.

The largest tsunami waves were observed in Indonesia’s Northern Sulawesi province, where waves reached a maximum height of around 0.83 metres on Monday morning, the tsunami warning centre said.

Indonesia issued and then cancelled tsunami warnings for Northern Sulawesi Gorontalo provinces, and the Sangihe Islands.

Officials in the Northern Mariana Islands and Guam also cancelled earlier tsunami warnings but said there was still a risk of strong currents and dangerous beach conditions.

A tsunami advisory remained in place for Japan’s southern coast and outlying islands, where residents were urged to stay away from river mouths and coastal areas until future notice.