US border police have found a sophisticated tunnel running between drug smugglers' warehouses in Otay Mesa, California, and Tijuana, Mexico Reuters

US border police have found a sophisticated drug smugglers' tunnel the length of six football fields linking southern California with Mexico and arrested two people.

The US customs and border protection agency said the tunnel ran between warehouses in Otay Mesa, California, and Tijuana, Mexico. It measured 1,800ft (550 metres) and was equipped with a rail system, lighting and ventilation.

Agents recovered more than 25 tonnes of marijuana in seizures related to the investigation in both California and Mexico and arrested a US citizen and his Mexican wife.

The couple were stopped at a border patrol checkpoint in southern California, driving a semitrailer loaded with 10 tonnes of marijuana. The truck had earlier left a warehouse that agents had under surveillance.

"This wasn't a mom-and-pop operation, or in this case a husband and wife operation. This is clearly organised crime at work. This was the cartels," said John Morton, director of the US Immigration and Customs Enforcement agency (ICE).

Mexico is in the grip of a drug war that has killed more than 30,000 people south of the border since December 2006, when President Felipe Calderon launched a crackdown on powerful drug gangs.

Mexican cartels have bored scores of tunnels under the US-Mexico border in recent years to beat ramped-up security at ports of entry and the rugged spaces in between. Nearly all of them linked cities on either side of Mexico's border with California and Arizona.

Investigators said the tunnel is about waist high and fitted with rails and a pulley system that they suspect was used to ferry drugs under the border in a cart. It had been in operation for less than a month.

In early 2006, agents discovered a tunnel measuring 2,400ft (731 metres) running under the same stretch of the border to Otay Mesa from Tijuana. It remains the longest such tunnel discovered to date.

Tijuana is the principal gateway for drugs entering California from Mexico. Last month, authorities seized more than 100 tonnes of marijuana valued at more than $340m in Mexico's biggest pot haul to date.

ICE is among several federal and local police agencies that comprise a special task force to search for tunnels running under the border in the San Diego area, which has soft, loamy soil that lends itself to tunneling.