They said seismic energy was once again building up inside the Guagua Pichincha volcano, 12 km outside the capital city, Quito, and in the Tungurahua volcano, in Ecuador's central Andes.

The threat of an explosion prompted authorities to suspend school on Tuesday for more than 400,000 students.

BBC's George Eykyn: Experts reckon there is an 80% chance of a full scale eruption

"There is a sustained build-up of activity," said Pablo Samaniego, a scientist with Ecuador's Geophysical Institute.

He said it was believed that a "major explosion" could occur "at any moment" in either volcano.

If there is an eruption of the Tungurahua volcano - the name means "throat of fire" in the local Indian language - hot lava could reach the central spa town of Banos within 15 minutes, local specialists calculate.

On Monday, the 5,016-metre (16,266ft) high volcano came back to life after a relatively calm Sunday, belching out rocks and ash for more than an hour.

Witnesses near Tungurahua, 125 km (78 miles) south of the capital, reported hearing explosions like "cannon shots" up to 15km (nine miles) away from the volcano.

"The rocks came out with force and a lot of noise," a witness told Agence France Presse news agency. "They broke apart as they fell to the ground."

"It seemed like the giant of the Andes threw out everything inside it," the witness said.

The volcano sporadically blew ash throughout Monday morning, with a plume at least 3,000 metres (9,900ft) high.

Authorities evacuated Banos's 17,000 residents on 16 October, along with 8,000 people living in half a dozen nearby villages.

Since then, they have used tear gas to prevent hundreds of people from returning to their homes to retrieve personal belongings and feed livestock they had left behind.

Two eruptions

The 4,000-metre Guagua Pinchincha had not erupted for 340 years until 7 October, when it showered Quito with about 15,000 tonnes of ash: the first of two major ash eruptions.

But despite the latest warning of an eruption, scientists do not believe Pinchincha will produce an explosion of lava.

The volcano, which has a crater facing away from the city, is not considered a direct threat to Quito's 1.4 million residents.