Disaster officials in Mexico noted 12 hours of temors at medium and high frequency, bursts of steam, ash and gas from Popcatepetl volcano Wednesday and Thursday, the National Center for Disaster Prevention said.
Officials at Benito Juarez International airport reported no ash over the airport, but American Airlines, Alaska Airlines, Air Tran, Delta Airlines, United Airlines and U.S. Airways all grounded flights out of precaution.
Canceled flights were between Mexico City and Houston, Dallas, Denver, Phoenix, Chicago, and Los Angeles.
Spirit Airlines also canceled flights between nearby Toluca International Airport and Ft. Lauderdale and Dallas.
Mexican airlines continued to operate on schedule as authorities established a 7-mile no-go zone around the peak.
Popocatepetl's last major eruption took place in 2000, but the volcano has been active since May and the alert level has been set one step below evacuation. Officials said they did not plan to raise the alert level, despite Thursday's increased activity.