Mexico’s violent drug cartels increasingly resemble an insurgency with the power to challenge the government’s control of wide swaths of its own soil, Secretary of State Hillary Rodham Clinton said Wednesday.

Clinton’s comments reflected a striking shift in the public comments of the Obama administration about the bloodshed that has cost 28,000 lives in Mexico since December 2006. They come as U.S. officials weigh a large increase in aid to the southern neighbor to help fight the cartels.

Clinton compared the conflict in Mexico to Colombia’s recent struggle against a drug-financed leftist insurgency that, at its peak, controlled up to 40% of that country. She said the United States, Mexico and Central American countries need to cooperate on an “equivalent” of Plan Colombia — the multibillion-dollar military and aid program that helped turn back Colombia’s insurgents.

“We face an increasing threat from a well-organized network, drug-trafficking threat that is, in some cases, morphing into, or making common cause with, what we would consider an insurgency,” Clinton said in response to a question after a speech at the Council on Foreign Relations in Washington.

As recently as last week, a senior State Department official staunchly denied that the drug war could be accurately described as an insurgency.

And although the administration has regularly praised the cooperation of Mexican authorities, some U.S. officials are beginning to show uneasiness about the partnership.

Top American officials have noted that the Mexican government does not always act on intelligence shared by the U.S., and some suspect corruption is sometimes the cause of the inaction.

“There is some frustration,” Alonzo R. Pena, deputy director of U.S. Immigration and Customs Enforcement, said in an interview. U.S. officials may pinpoint a particular house where a cartel figure is believed to be, and no operation ensues to capture him, he said.

Pena said that the Mexicans, who have lost an “astronomical” number of police officers and soldiers, may be simply cautious when they decide not to use U.S. information to attack the gangs. But at other times “it is completely corruption,” he said.

He said that he believed Mexican President Felipe Calderon and his close aides were trustworthy and committed to taking on the cartels. But U.S. officials are wary about cooperating with other elements of the Mexican government, fearing they can’t be trusted, Pena said.

The Calderon government quickly disputed Clinton’s assessment. Unlike Colombia, Mexico is acting “in time” to save its political system from being penetrated by the cartels, and to reform important institutions such as the police, the government’s spokesman on security matters, Alejandro Poire, said at a news conference.

“There is a very important difference between what Colombia faced and what Mexico is facing now,” Poire said. “Perhaps the most important similarity … is the extent to which organized crime and narcotics-trafficking organizations in both countries are fed by the enormous and gigantic U.S. demand for drugs.”

Foreign Minister Patricia Espinosa said in a radio interview that leftist rebels in Colombia had a political agenda, and established ties with organized crime to obtain resources. In Mexico, the cartels have no political agenda, she said.

Still, senior U.S. officials have grown increasingly alarmed in recent months at the expanding power and influence of the cartels, which now dominate swaths of the country. They battle one another and seek to cow Mexican citizens with violence that includes assassinations, beheadings and car bombings.

Authorities said Wednesday that Mexican marines had arrested seven gunmen suspected in the August massacre of 72 migrants from Central and South America, whose bodies were found on a small ranch near the town of San Fernando in Tamaulipas state. An additional suspect was captured earlier, and six have been killed in shootouts with authorities.

The seven are suspected of belonging to the Zetas cartel. They are thought to have kidnapped the migrants to force them to work as mules or fulfill other menial roles, and then allegedly shot them when they refused.

The two lead investigators in the case, who went missing a day after the bodies were discovered, have been found dead, officials said.

Also Wednesday, the mayor of a town in the relatively tranquil state of San Luis Potosi was gunned down in his office, the third Mexican mayor to be executed gangland-style in three and a half weeks. Alexander Lopez, 35, was shot to death midday by a man who burst into City Hall in the town of Naranjo, where Lopez had served as mayor for 11 months, local officials said. He was sitting at his desk when shot to death.

Although San Luis Potosi has not been engulfed in the same bloodshed as other states, Naranjo is located on the northeastern edge of the state bordering violent Tamaulipas. Intelligence sources say the Zeta cartel has been steadily moving into that part of the region.

There has been a growing outcry from officials in U.S. border states such as California, Arizona and Texas as the carnage has edged ever closer.

Some U.S. officials are questioning whether their Mexican counterparts are willing to stand up to the cartels as strongly as Colombian authorities. Clinton praised Calderon for his “courage and his commitment” but also called on Mexico to increase its “political will” to fight the cartels.

She said defeating the gangs will require stronger civil, police and military institutions, “married to political will, to be able to prevent this from spreading and beat it back.”

In Colombia, billions of dollars in U.S. aid and the policies of hard-line President Alvaro Uribe beat back the FARC rebels. Expanded police ranks have sharply reduced violent crime in the cities. Foreign investment has tripled, fueling a growing economy.

But Plan Colombia has drawn criticism for its heavy use of military force, the presence of hundreds of U.S military advisors and for human rights abuses. The program brought not only the military advisors, but also U.S. special forces personnel and a large numbers of defense contractors.

Clinton acknowledged that Plan Colombia was “controversial … there were problems and there were mistakes. But it worked.”

George Grayson, a specialist on Mexico at the College of William & Mary in Virginia, said Clinton’s remarks were a sign of U.S. officials’ growing alarm at the effects of the drug war.

He said that while President Obama didn’t even mention Mexico in his State of the Union message in January, more and more law enforcement and military officials see the situation as a top priority national security threat.

“It’s not like Afghanistan or Iran, but it’s suddenly on the national security radar,” he said.

Even so, he said he was skeptical that Mexico, with its nationalist sensitivities, would consent to a far more active U.S. role, even should Congress be willing to appropriate the funds.

Eric Olson of the Woodrow Wilson International Center for Scholars’ Mexico Institute said he senses from conversations with administration officials that “the administration still seems handcuffed by the lack of reliable partners at the operational level.”

Olson said that although he was reluctant to be alarmist, “I don’t think anybody thinks this has gotten to the bottom.”

Administration officials have said in recent days that despite the financial burdens of two other wars, they are considering a sizable increase in spending on the anti-drug war, as well as other improvements to the U.S. counter-narcotics security program.

A White House official who declined to be identified because he was not authorized to speak on the subject said last week that the joint effort with the Mexican government “remains a top administration priority.... We are constantly evaluating our efforts to make sure we are doing all we can on this issue.”

U.S. officials have been deliberating for some time how to follow up the Merida Initiative, a three-year, $1.6-billion program started in 2008 by President George W. Bush to provide equipment and training to the Mexican, Central American and Caribbean governments.

paul.richter@latimes.com

ken.dilanian@latimes.com

Staff writer Tracy Wilkinson in Mexico City contributed to this report.