Manila, June 5 : Authorities in the Philippines on Sunday raised the alert level for the Bulusan volcano after it spewed a grey plume about a kilometre high into the sky. The Philippine Institute of Volcanology and Seismology said the "phreatic eruption" occurred around 10.37 a.m., and lasted approximately 17 minutes. Ashfall was reported in towns near the volcano, located in Sorsogon province, southeast of Manila, Xinhua news agency quoted the Institute as saying. "Alert level one is now raised over Bulusan volcano, which means that it is currently in an abnormal condition," it said, warning of "increased possibilities of sudden and hazardous phreatic eruptions". The phreatic eruption is a "steam-driven explosion that occurs when water beneath the ground or surface is directly heated by hot rocks or new volcanic deposits or indirectly by magma or magmatic gas". Before the eruption, the Institute said it recorded 77 volcanic earthquakes in the past 24 hours. The institute prohibited people from entering the 4-km danger zone and urged residents of villages in the extended danger zone to be vigilant. "People living within valleys and along the river and stream channels, especially on the southeast, southwest and northwest sector of the edifice, should be vigilant against sediment-laden stream flows in the event of heavy and prolonged rainfall should a phreatic eruption occur," the Institute warned. It also warned airplanes to avoid flying close to the volcano's summit. Bulusan is one of the Philippines' most active volcanoes. In January 2018, the volcano spewed ash about 2.5 km high into the sky. /IANS