VILLAHERMOSA, Mexico - Gunmen murdered four relatives of a soldier who died last week in a shootout with drug traffickers that killed a major drug kingpin, in what government officials decried as an unprecedented revenge killing.

The massacre took place a few hours after the fallen soldier was buried Monday in Paraiso, in southeastern Tabasco state, the state's attorney general's office said.

Several gunmen "broke into the family home (of the soldier)... and strafed everybody inside with bullets," the office said in a statement.

The mother, brother, sister and aunt of late Navy infantryman Melquisedet Angulo Cordova were all killed. Another sister was wounded.

Though Mexico has grown used to reports of bloodshed, including gruesome decapitations and mutilations, over three years of violence that killed more than 15,000, Tuesday's execution-style murder of a family inside their home has outraged the country.

"It's a cowardly and contemptible act of violence," said President Felipe Calderon, who in 2006 sent 50,000 military reinforcements to help local police fight drug crime, especially in the northern regions bordering the United States.

"These outrageous actions show the lack of scruples of organized crime in mowing down innocent lives," he added during a public speech.

In the Gulf coastal city Paraiso, home to some 75,000 inhabitants, police, Army and Navy units were busy searching for the murderers of Cordova's family.

"We know there were several gunmen, but we lack details. The neighbours aren't talking. They're afraid," a Paraiso police spokesman told AFP by telephone.

Cordova was the only soldier killed last Wednesday in a shootout with drug traffickers in Cuernavaca that ended the life of "boss of bosses," drug kingpin Arturo Beltran Leyva, Mexico's third most-wanted man.

The firefight, in which six drug cartel members were also killed, was so intense that many residents at a block of flats in Cuernavaca had to be evacuated to a nearby sports stadium.

Leyva, 47, led a feared drug-trafficking operation of international reach and shared blame in drug violence in Mexico. A 1.5-million-US-dollar reward was posted for information leading to his arrest.

Leyva was tracked to Cuernavaca, 90 kilometers south of Mexico City, after a months-long hunt by authorities.

Ever defiant, Leyva's criminal organization on Tuesday let it be known it is not afraid of any government crackdown.

"It's not even the beginning of the war... you've made the terrible mistake of messing with THE business," read a sign in Cuernavaca in an apparent show of support for Leyva's alleged financier Edgar Valdez Villareal, whom authorities said could be the new "boss of bosses."

The United States believes the Beltran Leyva cartel has moved 5.8 billion US dollars worth of drug shipments in the past 18 years.