Philippine rescuers were scrambling to reach dozens of people feared buried under a building that collapsed a day after powerful earthquake north of Manila. The 6.3-magnitude quake struck northwest of the capital on Monday, scientists from the US Geological Survey said, heavily damaging Clark International Airport and sending terrified locals fleeing swaying high-rises. The worst of the damage was in the province of Pampanga, which was the site of most of the 11 fatalities, disaster officials said. Dozens of others were injured by falling rubble, including in Manila. The death toll could rise as crews fanned out across the region to assess damage in isolated villages that lost power and communications in one of the area's strongest tremors in years. Over 400 aftershocks have been registered since the initial earthquake - some as strong as 3.4 magnitude - Philippine seismologists said. Scores of rescuers in the town of Porac worked with cranes and excavators to peel back the concrete structure of a four-storey market building where up to 30 people were unaccounted for. "They [rescuers] still hear at least one person who is still alive," Pampanga Governor Lilia Pineda said. "This person is buried under concrete slabs." The earthquake also damaged several centuries-old churches which were crowded with worshippers in recent days as the majority-Catholic Philippines marked the Easter holiday. Father Roland Moraleja, who is based in Porac, said the 18th century belfry of Saint Catherine of Alexandria church collapsed in the earthquake. "It was the only part left from the old church," he said. "The historical value is now gone, but we are hopeful that it will rise again." High-rise buildings in the capital swayed after the tremor struck Monday evening, leaving some with large cracks in their walls. Video footage emerged of water from a penthouse swimming pool crashing down to streets below. Thousands of travellers were stranded after aviation authorities shut down the secondary Clark Airport, which is located on the site of the former US military base that lies about an hour's drive north of the capital. It was still closed on Tuesday as officials assessed the heavy damage to the terminal building and some cracking on the air traffic control tower. The earthquake was centred on the town of Castillejos, about 100 kilometres northwest of Manila, local geologists said. Dani Justo, a martial arts instructor, told AFP she was at her southern Manila home when the quake struck. "The clothes hanging on our line were really swaying. My shih tzu [dog] dropped flat on the ground," she added.. The government declared Tuesday a holiday for civil servants in Metro Manila to allow for safety inspections of buildings.