Obama and Mexican President Felipe Calderon vowed to tackle Mexico's violent cartels together during a symbolic visit of support by Obama south of the border, on the way to his first regional summit in Trinidad.

More than 7,000 people have died since the start of last year in violence between Mexican cartels and security forces that is spilling into the United States, and Obama followed a string of top US officials who have visited Mexico and admitted to a shared responsibility in Mexico's drug problem.

"I'm urging the Senate in the United States to ratify a treaty to curb" arms trafficking "that is a source of so many weapons used in this drug war," Obama told journalists here.

The treaty, a legally-binding regional agreement on illicit firearms trafficking, was adopted by the Organization of American States in 1997. It was never ratified by the United States.

Obama said he would not "pretend this is Mexico's responsibility alone. A demand for these drugs in the United States is what's helping keep these cartels in business. This war is being waged with guns purchased not here, but in the United States."

Mexican President Calderon, who has deployed tens of thousands of troops across the country to take on the cartels, said that the weapons were now aimed at Mexican authorities and society but that "organized crime is also being carried out in the United States."

Mexican authorities decommissioned 16,000 assault weapons in the past two years, Calderon said, adding that almost 90 percent of them were from the United States.

The US Bureau of Alcohol, Tobacco and Firearms estimates that around 90 percent of weapons confiscated in Mexico come from the United States.

Obama, welcomed by a sea of screaming schoolchildren waving US and Mexican flags on Thursday afternoon, said his first visit to Mexico marked the start of a new era of cooperation.

"I see this visit, as I know (Mexican) President (Felipe) Calderon does, as an opportunity to launch a new area of cooperation and partnership between our two countries," Obama said.

Obama and Calderon pledged cooperation in the face of the economic crisis, a message Obama was expected to take to regional leaders at an Americas Summit in Trinidad and Tobago, which starts on Friday.

The US president also reiterated his promise to push for comprehensive immigration reform, although the economic crisis has raised doubts over his plans to begin moving this year.

Mexico, Latin America's second biggest economy, depends on the United States -- where some 12 million documented and undocumented Mexicans live -- for around 80 percent of its exports and most of its remittances.

Calderon and Obama also announced plans to work together against climate change and to promote clean energy, although drug violence dominated the meeting.

On the eve of his visit, Obama slapped sanctions on three drug cartels and named a top US official to stiffen enforcement on the southern US border.

Last month he announced extra agents for the US border, and also vowed to staunch US demand for drugs.

In the latest violence, sixteen died in a shootout between suspected drug hitmen and soldiers in Guerrero in southwest Mexico late Wednesday, state prosecutor Eduardo Murueta said Thursday.