Mexican troops fanned out in the remote countryside near the Texas border on Thursday as they hunted the perpetrators of the worst massacre in the country's escalating drug war.

Heavily armed patrols in armoured personnel carriers, trucks and jeeps swept though towns and cities in the border region while helicopters buzzed overhead a day after the bodies of 72 people were found in an empty building at a remote ranch.

The victims, believed to be Central and South American migrants, appear to have been blindfolded and bound before they lined up against a wall and gunned down.

Photographs showed bloodstained bodies heaped on the ground at the ranch in Tamaulipas state, which has become the scene of some of Mexico's worst drug violence as the Gulf cartel and a spinoff group, the Zetas, fight over smuggling routes.

Officials said investigators were still examining the scene and had not yet removed the bodies.

Security forces killed three gunmen and arrested another when they approached the ranch on Wednesday, but several other suspects escaped during the fighting.

Migrants trying to slip into the United States are increasingly at risk of kidnapping and extortion by drug gangs that operate with near impunity in parts of northern Mexico, police and analysts say.

More than 28,000 people have died in drug violence since President Felipe Calderon launched his war on the cartels when he took office in late 2006.

Calderon has vowed to push ahead with the crackdown but has warned that more violence is likely ahead.

RESORT BLAST

While most of the bloodshed has been confined to gang members and security forces, violence is spreading to parts of the country once deemed peaceful.

At least five people were wounded in an explosion at a bar in the famed Mexican beach resort of Puerto Vallarta on Wednesday night, the Jalisco state prosecutor's office said.

Investigators believe the blast was intentional, but were unable to confirm media reports it was caused by a grenade, a spokesman for the prosecutor's office said on Thursday.

As many as 15 people were injured in the attack, including four who lost limbs, Mexican media said.

Puerto Vallarta is a popular Pacific coast destination for sun-seeking foreign tourists. It was not known if any foreign tourists were among the approximately 100 people in the bar when the attack occurred.

No motive for the attack was known although tourists are almost never targeted.

Violence in Jalisco, where Puerto Vallarta is located, has increased since security forces killed top drug trafficker Ignacio Nacho Coronel, who controlled the drug trade in the state, in July.

(Writing by Robert Campbell; Editing by Missy Ryan and Vicki Allen)

Mexico