The drug war has left more than 1,000 people dead in Mexico so far this year, with violence spilling over the border into the US.
"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"
Hillary Clinton, US Secretary of State
On Thursday Clinton toured the Mexican federal police's state-of-the-art headquarters, the key command centre in the country's bloody war on drugs.
Garcia Luna, Mexico's federal police chief, said Mexico was building, with US support, a modern police force capable of taking on organised crime.
Mexico has long complained its police force is often outgunned by drug dealers armed with firearms purchased in the US and smuggled into the country.
It is illegal to export guns to Mexico but US authorities rarely check vehicles or trains travelling across the joint border into Mexico.
|Mexico says its authorities are often outgunned by well-armed dealers [Reuters]
Analysts say the current US strategy to help Mexico fight the drug scourge, even though it is viewed as a largely Mexican problem, was a break from the previous administration.
Clinton said the administration of Barack Obama, the US president, wants to crack down on the sale of assault weapons that are arming the Mexican drug cartels.
In an interview with NBC television she said letting a previous US ban on the sale of assault weapons expire was "a mistake".
"I think these assault weapons, these military style weapons, don't belong on anyone's street," said Clinton, who had pushed for the ban as a New York senator.
"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."
Late on Wednesday Clinton had a private meeting with Felipe Calderon, the Mexican president, at his official residence.
Calderon praised the "first steps in terms of co-responsibility between the two countries in the fight against organised crime", according to a statement from the president's office.
Before wrapping up her visit, Clinton also urged young Mexicans to push for more democratic reforms and to fight corruption as part of a broader campaign against drug gangs.
She told students during a visit to Universidad TecMilenio that she hoped the "judicial and police reforms passed in recent months will come into full fruition".
"It's particularly important for the young people in Mexico, who have enormous power right now, to strengthen your democracy, to call for more reforms, to shine a bright light on corruption wherever you might see it."
Clinton said reform however was not enough.
"When one thinks about how important it is to tackle corruption and drugs… progress can only take hold if it is built on the foundation of economic growth and material improvement in people's daily lives," she said.
"We must demonstrate unequivocally that democracy produces positive outcomes."