The BBC's James Reynolds: 'Clouds of ash have covered parts of southern Quito'

About an inch of volcanic ash fell on the area on Wednesday as part of the Guagua Pichincha volcano - 12km (7.5 miles) away - began to crumble.

Quito and the surrounding area were placed on "orange alert" on Monday after scientists estimated there was a 90% chance of an eruption in the next few days or weeks.

Schools have been closed indefinitely, and emergency services are on high alert.

"My car was covered in a thick coat, almost an inch thick, of ash," said taxi driver Mauricio Narvaez.

President Jamil Mahuad said that although the developments were worrying, he would let scientists decide whether an emergency should be declared.

"We haven't thought of declaring a red alert. If the situation merits that, it will be up to the scientists," he said.

Seismologists have measured more than 2,000 earth tremors in the Quito area over the past few days, caused by an increase in volcanic activity.

"We saw the western wall of the dome crumbling when we made an aerial survey of the crater this morning [Wednesday] and we saw significant amounts of smoke," said Hugo Yepez, the director of Ecuador's main geological institute.

"The magma column is exerting enough pressure so it could deform the mountain," he added.

Villagers evacuated

The ash forced a state hospital in southern Quito to transfer or check out more than 100 patients.

"We can't function like this, the ash rain has stopped us from being able to work," the hospital's director said.

Ash and vapour has poured out of the volcano since August

The volcano has been sending ash and vapour more than three kilometres into the air since August.

Scientists said that if the volcano does erupt, it is likely that a large cloud of ash would disrupt communications and force Quito's airport to close.

About 2,000 residents from the nearby village of Lloa have been evacuated to the capital, although another 500 have stayed behind to look after their livestock.

Quito's 1.5m inhabitants have been urged to buy face masks and eye goggles and to ensure they have emergency supplies.

Guagua Pichincha last exploded in 1660, raining ash and rock on Quito but not spilling any lava.