Mexico's federal government has withheld 57 million pesos ($4.8 million) in security assistance for the violent border city of Ciudad Juarez, alleging it has failed to improve the quality of its law-enforcement agencies.

The subsidy from a fund to improve municipal public safety nationwide has been frozen, Sesnsp, the agency that coordinates the country's security policies, said in a statement.

That fund, known as the Subsemun, is one of three set up by the federal government in an effort to professionalize state and local police forces and equip them with the resources to take on heavily armed, well-funded drug mobs.

According to Sesnsp, Juarez authorities have not met a series of requirements for receiving the subsidy, including evening out officers' salaries and adopting a personnel-management system aimed at boosting professionalism.

This year, the government assigned Ciudad Juarez a total of 95 million pesos ($8.1 million) from the Subsemun, 38 million pesos ($3.2 million) of which has already been delivered.

From 2008 to 2010, that city of 1.2 million people only trained 145 police officers with Subsemun funds, equivalent to "just a 6 percent advance in the professionalization of (the municipal police personnel)," the statement said.

Over the same period, not a single command-level officer was trained in that city, whose homicide rate is among the highest in the world, according to the Sesnsp.

Local authorities also have failed to comply with a legal requirement to adjust their ordinances to the Mexican constitution and federal public safety laws, the agency added.

The announcement of the subsidy freeze comes amid growing tensions between federal authorities and local officials in that border city, where close to 5,000 Federal Police officers were deployed last year to tackle extreme levels of organized crime-related violence.

On Tuesday, the city's top cop, Julian Leyzaola, said at least 20 Federal Police officers shot at vehicles carrying him and his bodyguards in the wee hours of Tuesday as the chief was traveling to the municipal jail to quell a riot that left 17 inmates dead.

Federal Police said he had failed to stop at a checkpoint or identify himself.

But city clerk Hector Arcelus dismissed the federal cops' claim that the shooting was the result of confusion.

The federal officers are "perfectly" familiar with the vehicles used by Leyzaola and his escorts, according to Arcelus, who said it was "neither logical nor credible that they (the feds) acted randomly."

That same day, Mayor Hector Murguia, a member of the opposition PRI party, said he had reached an agreement with the federal government on the complete withdrawal from Juarez of the Federal Police.

The federal force had been deployed to that city across from El Paso, Texas, in April 2010 to take over law-enforcement duties from army soldiers, who faced numerous accusations of rights abuses.

President Felipe Calderon's administration, however, later denied that the Federal Police would be leaving the city, with interior minister Francisco Blake saying the efforts of the federal forces in Juarez have been "successful."

Tuesday's episode followed the killing by Federal Police in January of a member of Mayor Murguia's security detail at a checkpoint in Juarez.

The mayor, who demanded that the officer who shot his bodyguard be charged with homicide, subsequently got into a shouting match with the federal cops, who pointed their assault rifles at Murguia during a chance encounter in the city.

At least 14,000 "armed criminals" are in Ciudad Juarez and Chihuahua city, the capital of the likenamed state, working for the drug cartels that are fighting for control of smuggling routes into the United States, Chihuahua Attorney General Carlos Manuel Salas said last month.

About 5,500 of the armed criminals operating in Ciudad Juarez belong to Los Aztecas, a gang that works as the armed wing of the Juarez cartel, while the rest work for the Sinaloa cartel, Salas said.

More than 3,100 people were murdered in the border city last year, making 2010 the worst year since a war between rival drug gangs sent the homicide rate skyrocketing in 2008.

The murder rate remains high this year, with more than 1,000 people slain thus far in Juarez, but is down from 2010.