Washington - As the death toll in Mexico's drug wars soars into the thousands, the United States government and public have focussed more attention not just on drugs flowing north but also on the easy supply of weapons flowing south.

"There's no doubt that the vast majority of weapons seized in Mexico come from the United States," Attorney General Eric Holder said on a trip to Mexico earlier this month. "This is a reality we have to face in the United States, and it's one Mexicans have long had to confront."

The issue will be near the top of the agenda when President Barack Obama travels to Mexico Thursday ahead of the Summit of the Americas in Trindad and Tobago.

Obama's stop is meant to send a "very strong signal" to Mexican President Felipe Calderon that Obama "admires his work" in confronting the escalating violence and corruption, said Denis McDonough, deputy national security advisor for strategic communications, in a pre-trip briefing to reporters in Washington.

The US last month announced a huge boost in its cooperation with Mexican law enforcement officials to help bring down the illicit drug trade, and said it was doubling and even quadrupling its anti-drug efforts on the border.

The push includes a 400-million-dollar increase in previously paltry efforts to inspect cargo, trains and vehicles headed south.

Secretary of Homeland Security Janet Napolitano, herself the former governor of the border state of Arizona, said her department would send 360 more officers and agents to the border along with "cross-trained" dogs who can sniff out weapons and currency. More advanced X-ray systems to examine cars are also on the way.

"Our history has been focusing on goods and people coming north and what we're trying to do now is in addition to that, interrupt the flow of guns and cash south," she told Congress at a hearing last month at which she detailed new US efforts.

She called on Mexican authorities to join in with more inspections of cars going south.

"We need to get beyond getting lucky at a lane inspection," she said.

So far this year, US officials have seized hundreds of weapons headed into Mexico, with 997 found in just one week last month, Napolitano said.

A spate of Congressional hearings in recent weeks and the administration initiative show that the US government is beginning to take the issue more seriously, with concern that violence from Mexico will spill over into the United States.

Some lawmakers have called for restrictions on gun sales, including closing loopholes in laws requiring background checks for weapons bought at gun shows rather than from shops.

But gun laws are a politically sensitive issue in the US, where the Constitution guarantees citizens the "right to bear arms." Other lawmakers and the White House say the focus should be on enforcing existing laws and preventing criminals from getting guns to ship to Mexico.

On Sunday, Mexican Ambassador Arturo Sarukhan said in broadcast remarks that the US should reinstate its ban on assault weapons which expired in 2004.

Obama has steered clear of the politically explosive issue, insisting that enforcing current laws is the key to success. White House spokesman Robert Gibbs Tuesday warned that boiling down the US- Mexico security situation to just one issue would "oversimplify a complex problem."

Any US efforts by nature will be complicated, involving a massive bureaucracy across several departments and agencies, including the departments of Homeland Security, Justice and State, the Bureau of Alcohol, Tobacco and Firearms (ATF) and local and state police forces.

They face huge challenges. In 2008 a joint programme traced most of some 7,500 guns recovered by Mexican authorities to the US border states of Texas, California and Arizona. According to the ATF, 90 per cent of guns originated in the US.

The National Rifle Association, a gun rights group, points out that the number included only weapons traced under a programme designed to track US guns and excluded military-style weapons that originated in Central America or from within Mexico itself.

The most common weapons included 9mm pistols, 38-caliber revolvers, 5.7mm pistols,. 223-caliber rifles, 7.62mm rifles and. 50- caliber rifles, the ATF said.

"Trends indicate the firearms illegally crossing the US-Mexico border are becoming more powerful," the ATF said.

The ATF's own effort to stop firearms trafficking to Mexico, known as Project Gunrunner, referred 1,400 defendants for trial for 650 cases involving 12,000 arms last year.

That effort includes work to educate gun sellers to recognize smugglers, especially in states near the border where it is easier to buy guns.