Police in the US say they have arrested more than 300 members of a Mexican drug cartel and seized tonnes of drugs in the biggest operation of its kind.

The crackdown against the powerful La Familia cartel, which has fought for control of lucrative smuggling routes between Mexico and the US, was announced on Thursday by Eric Holder, the US attorney-general.

"This operation has dealt a significant blow to La Familia's supply chain of illegal drugs, weapons, and cash flowing between Mexico and the United States," Holder said.

More than 3,000 US federal agents and police officers were sent out across 19 US states over the past two days as part of the operation.

The deployment was part of a wider operation that has resulted in 1,200 arrests since 2005.

Mexican crackdown

Felipe Calderon, Mexico's president, has sent out 50,000 troops across his country in an attempt to stamp out drug gangs.

The La Familia cartel has control of drug production and distribution in the southwestern Mexican state of Michoacan and ships large quantities of cocaine and methampetamines to the US from there, US officials allege.

"The La Familia cartel has demonstrated an incredible level of sophistication and ruthlessness," Holder said.

"By seizing their drugs and upending their supply chains, we have disrupted their 'business as usual' state of operations."

Holder said US officer seized $3.4m in cash, 144 weapons, 109 vehicles, 729 pounds (31 kilograms) of methamphetamines, 137 pounds (62 kilograms) of cocaine and 967 pounds (439 kilograms) of marijuana during the series of raids.

"These are drugs that were headed for our streets, and weapons that often were headed for the streets of Mexico," he said.

La Familia's operations "stretch far into the United States," Holder said, calling the gang the "most violent" of the five main Mexican drug cartels.

Rising violence

A New York grand jury has indicted Servando Gomez-Martinez, the alleged chief of La Familia, and three other gang members on charges of conspiracy to import cocaine and methamphetamines.

If convicted, they face at least 10 years in prison and a maximum penalty of life imprisonment.

Holder's statement comes a day after a Mexican government report said that the murder rate in the Mexican city of Ciudad Juarez has reached record levels amid a continuing battle between two cartels.

The city, which lies close to Mexico’s border with the US has reported 1,986 murders - almost all drug-related - by the middle of October this year, the report from the Chihuahua state attorney-general's office said.

That marks a rise from the 1,171 murders in Ciudad Juarez for the same period in 2008, the report said.

There have been 195 murders this month alone, officials said.

According to Victor Valencia, Chihuahua state's public safety secretary, the killings, averaging seven a day in the city of 1.5 million, and can be blamed on an escalating war between the Sinaloa cartel run by Joaquin "El Chapo" Guzman and the Juarez cartel.

The cartels are fighting for control of smuggling routes between Ciudad Juarez and El Paso, over the US-Mexico border in Texas.