Lava is again flowing out of Mayon, the Philippines' most active volcano, raising fears an eruption could be imminent, authorities said Sunday.

The government has already evacuated around 63,000 people living inside a six-kilometre (3.7-mile) danger zone around the volcano, after it began to spew out white smoke and some lava last month.

Activity had appeared to quieten down but a fresh cascade -- confirmed by the Philippine Institute of Volcanology and Seismology Sunday -- this time stretching further down the slopes, has prompted concerns that an eruption may soon take place.

"The first activity started on September 15 and lasted for a few days. After that, there was a lull or no summit activity, but this morning, our volcanologists spotted a lava flow," said Renato Solidum, head of the government volcanology agency told ABS-CBN television.

"What is happening now is that there is very slow movement... of lava flow about 350 metres (1,148 feet) in length from the summit," he added.

Solidum warned lava flow from Mayon was usually followed by "an explosive phase of eruption" although he could not estimate when such blasts could occur.

He explained magma inside the volcano was now rising to the summit slowly but added that it could accelerate, prompting quakes and small explosions and potentially causing a much larger eruption.

The 2,460-metre (8,070-foot) Mayon, located about 330 kilometres (200 miles) southwest of Manila, has a long history of deadly eruptions.

Four foreign tourists and their local tour guide were killed when Mayon last erupted, in May 2013.

In 1814 more than 1,200 people were killed when lava flows buried the town of Cagsawa.

An explosion in August 2006 did not cause direct deaths, but four months later a typhoon unleashed an avalanche of volcanic mud from Mayon's slopes that killed 1,000 people.

The head of civil defence operations around Mayon, Bernardo Alejandro said the government had done an aerial survey of the volcano and cabinet ministers were in the area to assess the needs of people who had fled their homes.

He told AFP they would now be stricter in enforcing a ban on the entry of people into the danger zone.

Mayon is now rated on "level 3" meaning a possible eruption in weeks but Alejandro said they were now assessing whether to raise this to "level 4" meaning a possible eruption in days or even hours.