Scientists in the Philippines say the rumbling Mayon volcano has entered a more dangerous phase, belching a deadly "pyroclastic flow" of hot volcanic gas and ash.

Volcanologist Ed Laguerta said Sunday pyroclastic flows are more worrying than normal lava flows because they cover wide areas and can race down the volcano's slopes at speeds of at least 60 kilometers per hour.

Volcanologists last week said the volcano's level of sulfur dioxide emissions had dropped, which could mean pressure is building up for an explosive eruption.

Nearly 40,000 people have left their homes and farmland for emergency shelters as they wait to see what the 2,400-meter volcano will do.

Mayon is one of the Philippines' most active volcanos. It has erupted at least 47 times in the past 400 years.

Some information for this report was provided by AFP and AP.