MEXICO CITY - U.S. airlines canceled more flights in and out of Mexico City's airport on Friday as a precaution as the nearby Popocatepetl volcano continued to emit vapor and ash.

Alaska Airlines, United, Delta and AirTran canceled about a dozen flights, fewer than the number on Thursday, said Jorge Gomez, spokesman for Mexico City International Airport.

He noted that the airlines made the decision, and said normal operations continued at the airport without restrictions. No ash has fallen at the airport, Gomez said, though dust particles have been detected from the volcano that is about 40 miles away.

At least six U.S. airlines canceled more than 40 flights on Thursday as the volcano spewed a mile-high plume of ash that drifted over large parts of Mexico City. The volcano also spewed a hot shower of glowing rock around its crater.

Mexico's National Center for Disaster Prevention reported that there had been 99 tremors and exhalations of medium and high frequency from the volcano on Friday afternoon.