The Philippines raised the alert level at its Mayon volcano on Monday after a loud explosion in the wake of increased activity made a hazardous eruption likelier, prompting authorities to close all schools and urge residents to stay indoors. <br/><br/>Mayon, the country's most active volcano, has been spewing ash, lava, and pyro-clastic material since Jan 13, displacing close to 40,000 residents in the central province of Albay. <br/><br/>The Philippine Institute of Volcanology and Seismology raised its alert on Mayon to level 4, signifying a hazardous eruption is imminent, from level 3, which warns that such an eruption could be weeks or even days away.<br/>Agency Chief Renato Solidum said in a news conference, the danger zone around the 2,462-metre volcano has been expanded to a radius of eight kilometres. He added that a level 5 alert signifies a hazardous eruption is underway.