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.

With helicopters overhead, heavily armed patrols in armoured personnel carriers, trucks and jeeps swept though towns and cities in the border region 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, appeared to have been blindfolded and bound before they were 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.

The sole survivor of the massacre, an unidentified Ecuadorean man, escaped the remote ranch on Monday after being shot and told authorities about the ordeal. He said his fellow victims included Brazilians, Costa Ricans, Hondurans, Salvadorans and Guatemalans.

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

Security forces were fired upon when they approached the ranch on Tuesday, and in the ensuing firefight marines killed three gunmen and arrested another. Several suspects escaped.

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

Ruling party politicians defended the government's crime record on Thursday, saying the massacre signalled desperation within criminal groups under pressure from the government.

But activists denounced the government for failing to protect migrants, saying thousands of undocumented travellers suffer abuse at the hands of criminal gangs every year.

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

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

Investigators initially though the blast was intentional but later arrested five of the wounded on suspicion they accidentally caused the explosion.

Puerto Vallarta is a popular Pacific coast destination for sun-seeking foreign tourists but the bar is located in a residential neighbourhood away from the main tourism district.

No foreigners were injured in the blast, the prosecutors' office said.

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