Aftershocks rocked the northern Philippines early Wednesday, hours after a strong earthquake injured at least six people and caused substantial damage to a hospital and several old churches, authorities said.

The 6.4-magnitude quake struck the mountain town of Dolores in Abra province late Tuesday, followed by numerous aftershocks over the rest of the night, the state seismology office said.

"We hid under a table and my family only went out of the house after the shaking stopped," Abra rescuer Ron Sequerra told AFP by telephone, adding his family had been woken by strong ground shaking.

Six people were injured in the Abra town of Lagayan, Sequerra added.

The Lagayan mayor's office and a high school building were sealed off after they sustained cracks and broken glass windows, according to pictures posted on the town's official Facebook page.

In the city of Batac in the neighboring province of Ilocos Norte, several patients spent most of the night outside a government hospital after the ceiling collapsed on several rooms and damaged equipment, hospital staff said.

Boulders rolling down a hillside temporarily blocked a road linking Batac to the nearby town of Banna, but rescue officials said the landslide had since been cleared.

A number of old churches in Abra and Ilocos Norte also sustained damage, the civil defense office said.

Ilocos Norte governor Matthew Manotoc declared a school holiday and government workers were told not to report for work as the authorities inspected the integrity of buildings.

In July, a 7.0-magnitude quake also in mountainous Abra province triggered landslides and ground fissures, killing 11 people and injuring several hundred others, according to the official count.

Quakes are a daily occurrence in the Philippines, which sits along the Pacific "Ring of Fire", an arc of intense seismic as well as volcanic activity that stretches from Japan through Southeast Asia and across the Pacific basin.