Mexico's navy has seized more than a tonne of cocaine stuffed inside frozen sharks, as drug gangs under military pressure go to greater lengths to conceal narcotics bound for the United States.

Armed and masked navy officers cut open more than 20 shark carcasses filled with slabs of cocaine after checking a container ship in a container port in the southern Mexico state of Yucatan, the navy and Mexican media said yesterday.

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"We are talking about more than a tonne of cocaine that was inside the ship," Navy Commander Eduardo Villa told reporters after X-ray machines and sniffer dogs helped uncover the drugs. "Those in charge of the shipment said it was a conserving agent but after checks we confirmed it was cocaine," he said.

Drug gangs are coming up with increasingly creative ways of getting drugs into the United States - in sealed beer cans, religious statues and furniture - as Mexico's military cracks down on the cartels moving South American narcotics north.

President Felipe Calderon has sent 45,000 troops and federal police across Mexico to try to crush powerful smuggling cartels. But traffickers armed with a huge arsenal of grenades and automatic weapons are far from defeated, worrying Washington as violence spills over into US states like Arizona.

Some 2,750 people have died in drug violence in Mexico this year, a pace similar to that of 2008, when 6,300 were killed.

Led by Mexico's most wanted man, Joaquin "Shorty" Guzman, smugglers from the Pacific state of Sinaloa are fighting a turf war with rivals. Guzman seeks to control Mexican and Central American smuggling routes into the United States.