Hillary Clinton and Mexican President Felipe Calderon met in Mexico City
US Secretary of State Hillary Clinton is pledging further effort to help Mexico in its anti-drugs campaign.
Mrs Clinton said the use of military-style assault weapons was a particular concern, and she would discuss reimposing a ban on their sale.
A previous US decision to lift a ban on such sales had been a mistake, she told the NBC television network.
Earlier on her visit she admitted that America's appetite for drugs was helping to fuel the violence.
Some 8,000 people have died in drug-related violence in Mexico over the past two years.
Mrs Clinton said assault weapons did not belong on anyone's street.
MEXICO: FAILED STATE?
Enrique Krauze, historian and editor of Letras Libres, Mexico
A widespread idea, especially in the US, is that Mexico is, or is on the verge of becoming, a "failed state". This is not only wrong but a caricature of reality.
It is true that Mexico has a serious problem now in the progress of the war, a just war, unleashed by the Calderon government against the well-armed, vicious and extremely wealthy drug lords of Mexico.
But Mexico is a country with very strong institutions, made even stronger by the birth of true democracy in 2000, when the 70-year one-party rule of the Institutional Revolutionary Party ended.
We're a country that has overcome a number of serious crises: financial, governmental and electoral, and we have emerged stronger in every case because of our effective responses.
This war can only be won by reducing the range of the drug gangs and the flow of illegal guns from, and demand for drugs within, the US.
Mexico as a nation does not live in fear but we are deeply worried. The country will not collapse but Mexico and the US have to work together on these problems. There is no need for despair, but for action, in both countries.
A version of this commentary appeared in the New York Times. Translation by Hank Heifetz
She was referring to a ban on the sale of such powerful guns which was in effect in the United Sates between 1994 and 2004.
"During that time," said Mrs Clinton, "police in America were able to drive down crime because they didn't have to worry about these assault weapons getting into the hands of criminals and gang members.
"So we will make the case that we need to put more teeth in the law, try to prohibit the sale outside of our borders of these guns," she said, stressing that Congress might oppose such a ban.
On the first day of her visit, Mrs Clinton had said an "insatiable" US demand for illegal drugs was fuelling the drug trade.
"Our inability to prevent weapons from being illegally smuggled across the border to arm these criminals causes the deaths of police officers, soldiers and civilians.
"I feel very strongly we have a co-responsibility."
She also acknowledged that US efforts to ban drugs had so far been unsuccessful in stopping the narcotics trade.
"Clearly, what we have been doing has not worked and it is unfair for our incapacity... to be creating a situation where people are holding the Mexican government and people responsible," she said.
Mrs Clinton said the Obama administration, working with the US Congress, intended to pledge $80m (£55m) to help Mexico buy Blackhawk helicopters.
"These aircraft will help Mexican police respond aggressively and successfully to the threats coming from the cartels," she said.
The BBC's James Coomarasamy spoke to Janet Napolitano about US- Mexico security
Mrs Clinton's trip is the first in a series of visits by high-level officials, including Homeland Security Secretary Janet Napolitano and Attorney General Eric Holder, before President Barack Obama himself visits Mexico in mid-April.
Ms Napolitano told the BBC on Wednesday that there had been a significant escalation of violence in Mexico, in part because of US efforts to clamp down on trafficking routes.
But, she said: "The most important thing is that the federal government of Mexico is now battling these cartels, and they weren't in the past. And as a result the violence between the cartels and the government of Mexico has really increased."
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