Two powerful offshore earthquakes struck off the southern Philippines on Friday, killing at least seven people, while towns near the epicentre suffered structural damage and authorities warned of strong aftershocks.

The first quake of magnitude 7.4, in waters off the town of Manay in the province of Davao Oriental, triggered a tsunami alert for coasts within 300 kilometres of the epicentre, but the warnings for the Philippines and Indonesia were subsequently lifted.

A second earthquake of magnitude 6.8 struck the same area seven hours later, triggering a new tsunami warning, with the country’s seismology agency Phivolcs warning of possible waves that could be more than a metre higher than normal tides.

People living near coastal areas in the southern Philippines were “strongly advised to immediately evacuate” to higher ground, or move further inland, it said in an advisory.

Phivolcs Director Teresito Bacolcol described the twin earthquakes as a “doublet”, two distinct earthquakes that occurred along a massive trench off the country’s eastern seaboard.

There were initial reports of damage to homes, buildings and bridges, one official in Manay said, although the full extent of the damage in the Philippines was not immediately clear.

At least seven people were killed, civil defence official Ednar Dayanghirang said in a briefing. The fatalities were reported in towns and cities near the earthquake’s epicentre.

The twin quakes were among the strongest in recent years to hit the Philippines, which sits on the Pacific “Ring of Fire” and experiences more than 800 quakes each year.

It came two weeks after the Philippines experienced its deadliest earthquake in more than a decade with 74 people killed on the central island of Cebu following an offshore quake of magnitude 6.9.

Vehicles shaking, gates rattling

A video posted on social media and verified by Reuters showed people in the city of Davao calmly holding on to parked vehicles that rocked from side to side as the ground shook, as metal gates rattled nearby.

Richie Diuyen, a disaster official in Manay, said the quake lasted 30 to 40 seconds and damaged some homes and the facade of a church, while leaving cracked roads and unpassable bridges.

“We couldn’t stand earlier. I am 46 years old now, and this is the strongest earthquake I ever felt,” Diuyen said by phone.

Earlier, President Ferdinand Marcos Jr said authorities were assessing the situation and search-and-rescue teams would fan out once it was safe.

“We are working round the clock to ensure that help reaches everyone who needs it,” he said in a statement.

Warning of strong aftershocks

Phivolcs warned of aftershocks and urged people in affected areas to stay away from the shoreline. Verified footage from the southern Philippines showed workers filing out of buildings to gather in the streets, lamps swaying in shops, and offices with toppled cabinets and workers holding on to desks as structures and fittings around them creaked.

Video images from Indonesia’s island of Sulawesi showed fishing boats returning from sea and children playing on a beach from which water had receded.

Phivolcs revised down the magnitude to 7.4 from an initial figure of 7.6, and put the quake’s depth at 23km. The second quake was also downwardly revised to 6.8 from 6.9, with an depth of 37km.

The governor of Davao del Norte in the Philippines said people panicked when the earthquake struck.

“Some buildings were reported to have been damaged,” Edwin Jubahib told broadcaster DZMM. “It was very strong.”