Seismic activity has increased at the Popocatepetl volcano near Mexico City, leading authorities to alert towns in two central states and the capital.Mexico's National Disaster Prevention Center says the white-capped volcano spewed a plume of steam more than a half mile (1 kilometer) into the sky. The volcano shook during Saturday night, sometimes emitting glowing rock over the crater.The government deployed soldiers and federal police to the area Sunday in the event of a bigger eruption, and officials closed off a seven-square-mile (18-square-kilometer) zone around the cone of the 17,886-foot (5,450-meter) volcano. State authorities prepared shelters.Popocatepetl has put out small eruptions of ash almost daily since a round of activity began in 1994. The eruptions started strengthening two weeks ago and have increased even more this weekend.