WASHINGTON (Reuters) - Drug gangs in Mexico and Central America are morphing into an insurgency like that which gripped Colombia 20 years ago, Secretary of State Hillary Clinton said on Wednesday, promising more U.S. help to fight traffickers.

“These drug cartels are now showing more and more indices of insurgency. All of a sudden, car bombs show up, which weren’t there before,” Clinton told a foreign policy think-tank in Washington.

“It’s looking more and more like Colombia looked 20 years ago, where the narco-traffickers control certain parts of the country,” Clinton said.

Mexico’s President Felipe Calderon deployed thousands of army and federal police to attack cartels but violence is still spiraling out of control in some cities near the U.S.-Mexico border as cartels battle for smuggling turf.

More than 28,000 people -- mostly traffickers and police but also politicians and children -- have been killed since Calderon took office in late-2006, undermining Mexico’s global image and threatening a plodding economic recovery.

On Wednesday, the mayor of El Naranjo, a small town in the central state of San Luis Potosi, was killed in his office in front of several colleagues, local media reported.

The United States has already pledged some $1.4 billion over three years to Mexico in a thus-far unsuccessful effort to crush cartels who smuggle $40 billion worth of cocaine, heroin, amphetamines and marijuana into the United States each year.

The Obama administration has promised additional help, including steps to stop guns flowing southwards -- a major source of arms for traffickers -- and to address illegal drug demand in both countries, an underlying cause of the crisis.

INSURGENCY EMERGING

Despite these efforts, Clinton said drug gangs were “in some cases morphing into, or making common cause with, what we would consider an insurgency in Mexico and Central America.”

“We are working very hard to assist the Mexicans in improving their law enforcement and their intelligence, their capacity to detain and prosecute those who they arrest,” Clinton said.

But she said that Central American and Caribbean leaders were increasingly voicing the same concerns, and in some cases lacked the institutional capacity to fight back.

Mexico’s government disagreed with Clinton’s statement and said it was taking prompt steps to curb the power of cartels.

“We certainly don’t share that stance. There are very important differences between what Colombia faced and what Mexico is facing now,” senior national security official Alejandro Poire told a news conference on Wednesday.

Poire said the main similarity between Colombian and Mexican traffickers is that both are fed by the constant demand for illegal drugs in the United States.

Clinton cited the success of “Plan Colombia”, which channeled billions of dollars in U.S. funds to the Colombian government to help beat back FARC rebels which financed their activities through the lucrative cocaine trade.

Once a powerful force capable of controlling large swaths of the country, FARC has been seriously weakened after a string of desertions prompted by government bounties and improved military intelligence.

Clinton conceded that Plan Colombia -- which critics said sometimes involved human rights abuses by Colombian security forces -- was controversial but said the improved conditions in the country now were a direct result.

“There were problems and there were mistakes but it worked,” Clinton said. “We need to figure out what are the equivalents for Central America, Mexico and the Caribbean, and that’s not easy.”