Four US airlines canceled at least 10 flights to and from Mexico City's airport Friday after the Popocatepetl volcano spewed more ash, a day after hundreds of air passengers were stranded for the same reason, an airport spokesman said.

The spokesman said United Airlines, Delta Airlines, Alaska Airlines and AirTran voluntarily scrapped the flights as a precaution, but the airport was otherwise operating normally.

United canceled its flights to and from Los Angeles and the Texas city of Houston, but there was no immediate information about the other airlines or the number of new passengers affected, the spokesman said.

The new cancelations came one day after six US airlines called off 60 flights due to volcanic ash, leaving some 600 people waiting for new flights home on Friday morning, he said.

In its latest bulletin, the National Disaster Prevention Center said the 5,452-meter (17,887-feet) high volcano had blown a 1.5 kilometer (one mile) high column of ash that was heading west-northwest and produced low-intensity tremors in the last 24 hours.

Volcanic activity at the snow-capped Popocatepetl, located 65 kilometers (40 miles) southeast of the capital, has intensified since May, prompting authorities to raise the alert level to "yellow phase 3" for 27 days, just short of evacuation orders.

The disaster prevention center advised people living around the volcano to wear long sleeves and use masks or cover their mouths and noses with a cloth.