|Clinton has likened the spate of drug gang-related violence in Mexico to the situation in Colombia [Reuters]
Masked armed men have shot dead the mayor of a small town in northern Mexico in the latest attack believed to be linked to drug gangs.
Prosecutors said four hooded men pulled up outside the office of Alexander Lopez Garcia in El Nranjo in the state of San Luis Potosi on Wednesday, before two of them burst in and shot him to death.
The office of Felipe Calderon, the Mexican president, issued a statement saying he "energetically condemned" the killing.
El Nranjo, the rural township of about 20,000 people, borders the state of Tamaulipas where 72 migrants were massacred by drug gang members in August.
Against this backdrop, Hillary Clinton, the US secretary of state, has said Mexico is "looking more and more like Colombia looked 20 years ago, where the narco-traffickers control certain parts of the country, not significant parts".
Speaking at the Council on Foreign Relations in Washington, she said drug gangs are "morphing into, or making common cause with, what we would consider an insurgency in Mexico and in Central America".
"These drug cartels are now showing more and more indices of insurgency," Clinton said. "All of a sudden, car bombs show up which weren't there before."
Clinton also suggested that "we need to figure out what are the equivalents" for Mexico and Central America of the US Plan Colombia - in which American special forces teams train Colombian troops and US advisers are attached to Colombian military units.
Mexico has long rejected allowing US troops on its soil.
Clinton's comments at the raised hackles in Mexico.
"Of course we do not agree with the statement in this regard, given that there are very important differences between what Colombia faced then and what Mexico faces today," Alejandro Poire, a Mexican government security spokesman, said.
Later, Arturo Valenzuela, the US assistant secretary of state, said Clinton's comments should not be misinterpreted.
"What we are concerned about is the fact that you see the development of phenomena of car bombs that can affect innocent people and these are terrorist acts, you can define them as terrorist acts," he said.
"But the term insurgency should not be viewed in the same way we would refer to a Colombian insurgency. Not an insurgency of a militarised group within a society that is attempting to take over the state for political reasons."
Valenzuela said Mexico was facing an escalation of violence by criminal organisations, and not "an insurgency".
Garcia's killing on Wednesday is the third such incident in the past month, with police suspecting drug gangs to be behind the attacks.
The mayor of a town just across the state line in Tamaulipas was shot to death and his daughter wounded on August 29.
The mayor of Santiago, a town in the neighbouring state of Nuevo Leon, was found murdered on August 18.
In another development, prosecutors announced the arrest of seven suspects in the August massacre of the 72 migrants in Tamaulipas.
Four of the suspects were arrested after a September 3 gun battle with soldiers, while the other three were captured days later, Alejandro Poire said at a news conference.
The arrests "will help determine exactly what happened in San Fernando, Tamaulipas, and it's a significant step toward ending the impunity surrounding assaults on migrants by organised crime", he said.
Investigators believe the migrants were kidnapped by the Zetas and killed after refusing to work for the drug gang.