WASHINGTON (Reuters) - Powerful groups in the United States appear to be blocking efforts to stem the flow of assault weapons fueling Mexico’s drug war, Mexican President Felipe Calderon said in an interview broadcast on Sunday.

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Calderon, who has deployed tens of thousands of soldiers and police to fight drug cartels, told Fareed Zakaria’s “GPS” program on CNN that there was resistance in Washington to Mexico’s demands that sales of such weapons be stopped.

“They (U.S. officials) say that they are facing strong opposition and there is powerful lobbies in the Congress in order to change that situation,” Calderon said in a pre-taped interview in Mexico City.

The Mexican leader added that solving the cross-border gun trafficking problem was critical to his bid to crack down on the drug-related violence that has killed 4,600 people in the past two years.

Mexico says 90 percent of the weapons used by drug gangs are bought in the United States, often legally. Mexican officials also want to see the U.S. Congress reinstate a ban on the sale of assault weapons that expired in 2004.

U.S. gun rights groups generally oppose such a restriction.

The United States is already deeply involved in Mexico’s struggle with drug gangs and has pledged some $1.4 billion over three years in a thus-far unsuccessful effort to crush cartels who ship $40 billion worth of illegal drugs north each year.

But concerns the violence in Mexico is escalating -- two U.S. citizens were shot to death this month in the violent Mexican border city of Ciudad Juarez -- has led top U.S. officials to pledge more assistance.

U.S. Secretary of State Hillary Clinton led a high-level delegation to Mexico City last week for talks, underscoring the Obama administration’s concerns about the drug violence south of the border.

Washington has started to increase searches of southbound vehicles on its border with Mexico for guns and money heading to Mexican cartels.