President Obama pledged Thursday that the U.S. would become a better partner in curbing the flow of arms that have aggravated a bloody drug war in Mexico, but acknowledged that political realities make it tough for him to ban some of the most potent weapons in the arsenals of drug cartels.

Emerging from a meeting with Mexican President Felipe Calderon, Obama said he favored a ban on assault weapons but would not push to reimpose a U.S. prohibition that lapsed in 2004.

“None of us is under any illusion that reinstating that ban would be easy,” Obama said at a news conference after talks that dealt in part with the violence that has swept sections of Mexico.

Instead, he announced plans to increase the number of U.S. law enforcement personnel at the border to search for smuggled shipments of guns, even in southbound trains. He also said he would push the Senate to ratify a decade-old treaty on arms trafficking as part of a concerted U.S.-Mexican effort to defeat drug gangs.

But despite Obama’s high approval ratings and solid Democratic majorities in the House and Senate, his comments indicated that the political clout of gun rights advocates, including many Republicans as well as conservative Democrats, made it doubtful he could resurrect an assault gun ban.

Congress enacted such a ban in 1994, but it expired after 10 years. In 2004, when Sen. Dianne Feinstein (D-Calif.) proposed an extension, it was voted down, 90 to 8.

Mexican officials have made it clear they want the ban reenacted. But Obama, as he stood beside Calderon, said other measures would have to suffice.

When it was his turn to answer the assault weapons question, Calderon struck a patient tone and said he grasped the nuances involved. His government has seized 16,000 assault weapons since he took office in December 2006. The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives says 90% of weapons seized in Mexico and reported to the agency can be traced to the United States.

“We understand that this is politically very sensitive because we know the great esteem Americans have for their constitutional rights, especially those contained in the 2nd Amendment,” Calderon said.

But he cautioned that the widespread violence plaguing Mexico may spill into the U.S.

“These weapons today are aimed at Mexican authorities and Mexican citizens, but organized crime is not only present here in Mexico. It’s also in the United States,” he said. “I hope to God these weapons that today are sold in the U.S. and used in Mexico are not one day also used against U.S. society and U.S. authorities the way they are here in our country.”

Illustrating the dangers, a gun battle on the eve of Obama’s arrival left one soldier and 14 alleged drug traffickers dead in the Pacific coast state of Guerrero, officials said. About 30 gunmen attacked troops who were patrolling a remote mountain ridge. A second soldier was critically wounded.

Increasingly brazen traffickers have started attacking army patrols head-on. Authorities said the Mexican military, after the battle, confiscated a small arsenal, including two .50-caliber Barrett rifles, 17 other rifles, grenades and ammunition and eight vehicles.

Obama’s stop in Mexico was scheduled to last less than 24 hours and was made en route to a summit in Trinidad and Tobago of 34 Western Hemisphere nations.

There, Obama is likely to face criticism for the long-standing U.S. trade embargo against Cuba. Calderon said the embargo was “not very useful” in promoting change. “It was implemented before President Obama and I were born,” he said. “And things in Cuba have not changed much.”

Obama has lifted restrictions on Cuban Americans who want to travel to Cuba. But he opposes lifting the trade embargo, calling it useful leverage in getting Cuba’s rulers to adopt democratic reforms.

The summit will be attended by many staunch U.S. critics, given that Latin American nations have leaned leftward in recent years. The White House said Obama was not likely to have a one-on-one meeting with one of America’s harshest critics, Venezuelan President Hugo Chavez.

A purpose of Obama’s visit Thursday was to show support for Calderon after warnings from U.S. military and intelligence officials that drug cartels pose a mortal threat to the Mexican government. Mexico objected to such alarm, and the Obama administration has been working to make amends.

As the first U.S. president to visit Mexico’s capital in 12 years, Obama is delivering a message that he appreciates the courage shown by Calderon in combating drug lords, White House officials said.

At a welcoming ceremony at Los Pinos, the Mexican equivalent of the White House, Obama said, “At a time when the Mexican government has so courageously taken on the drug cartels that have plagued both sides of the border, it is absolutely critical that the United States joins as a full partner in dealing with this issue.”

Calderon’s government has deployed 45,000 soldiers to parts of the country beset by drug violence. For its part, the Obama administration has pledged to intensify border patrols and speed up shipments of military aircraft to help Mexico suppress drug gangs.

Not all Mexicans share Obama’s opinion of Calderon. The Mexican leader has been criticized for underestimating how deeply drug gangs have corrupted local governments and police forces. Critics also contend that Calderon is relying too heavily on military force while neglecting politically sensitive areas that should also be addressed, such as money laundering, judicial reform and high-level corruption.

Calderon’s use of military force also has led to accusations of human rights abuses. Mexico’s National Human Rights Commission has said citizens’ complaints of killings, rape and other abuses have grown sixfold since Calderon assigned the army to the drug war shortly after he took office.

Even if Mexico was left disappointed on assault weapons, the two leaders stressed that they had found common ground on other topics.

Before they met, the White House announced the countries had agreed to work jointly to curb global warming and develop clean energy alternatives.

peter.nicholas@latimes.com

wilkinson@latimes.com