Travel plans of thousands of passengers were disrupted for several days after four more U.S. air carriers canceled at least 10 flights from Mexico City as the country's Popocatepetl Volcano continued its activity.

United Airlines, Delta Airlines, Alaska Airlines and AirTran had voluntarily suspended their flights as precautionary measures since Saturday. On Friday, six U.S. airlines stopped 60 flights because of the volcanic ash that spewed 2 miles high.

The continuous volcanic activity of Popocatepetl, located halfway between Mexico City and Puebla, led Mexico's National Center for Disaster Prevention to raise the volcano alter to Stage 3 Yellow from Stage 2 Yellow on Saturday.

The center said the ash plume is heading west-northwest and producing low-intensity tremors in the past 24 hours. The Stage 3 Yellow alert is short of evacuation orders.

However, the centre advised residents living near the volcano to wear long sleeved tops and used masks or cloth to cover their mounts and noses.

Fausto Lugo, the civil protection secretary of Mexico City, said in the event of a full blown eruption, they would not evacuate Mexico City but focus on managing the handling of volcanic ash so it would not pose a risk to the health of residents or prevent it from mixing with potable water and form concrete-like substance that block drains.