Mexico drug crackdown nets 500

Police chief Alejandro Dominguez was gunned down in Nuevo Laredo

Mexico has held nearly 500 people in an ongoing operation against drug-related violence, the government says.

Authorities say they have confiscated more than four tonnes of marijuana in the first week of the offensive by Mexican forces along the US border.

The focus is Nuevo Laredo, where about 700 police are being investigated over alleged roles in drug-related crime.

The city is a key transit point for drugs into the US and cartels have been fighting for control of the market.

A BBC correspondent in Nuevo Laredo says this weekend saw the 71st death this year from the violence.

City chief shot

The Mexican army and federal police are deployed in the states of Baja California, Sinaloa and Tamaulipas.

Mexico has asked the US to boost border security to stop weapons-trafficking, following criticism from Washington over the way Mexico manages security.

US Secretary of State Condoleezza Rice said the Bush administration was increasingly concerned about drug-related violence along the Mexican border.

Concerns about lawlessness in Nuevo Laredo were reignited earlier this month after the city police chief was shot dead hours after he took up the post.

More than 500 people are said to have died so far this year in the violence in northern Mexico.