The 550-metre-long tunnel discovered in Otay Mesa was allegedly used to traffic drugs by cartels [Reuters]

US border police say they have have discovered a massive, sophisticated tunnel linking Mexico with neighbouring southern California that is believed to have been used by cartels to traffick narcotics.

Authorities said on Wednesday that they had seized more than 25 tonnes of marijuana after police searched the 550-metre tunnel on both sides of the border. The tunnel descended around six metres underground before turning and heading under the US-Mexico border fence.

According to US officials, the tunnel, which is length of six football fields and equipped with a rail system, lighting and ventilation, connected warehouses in Otay Mesa, California, with others in Tijuana, Mexico.

The multi-agency San Diego Tunnel Task Force, which is led by US Immigration and Customs Enforcement (ICE), discovered the tunnel after agents followed a truck driving suspiciously from an industrial area near the Otay Mesa border crossing. The truck, with a husband and wife inside, was stopped at a US Border Patrol checkpoint and found to have around 10 tonnes of marijuana packed into the 53-foot trailer.

"This wasn't a mom-and-pop operation, or in this case a husband and wife operation. This is clearly organized crime at work. This was the cartels," said ICE director John Morton. 

Authorities estimated the worth of the drugs at around $20 million. The 10 to 15 tonnes of marijuana discovered in the warehouse were organised in barrels and wrapped in cellophane, as if ready to be shipped, the Los Angeles Times newspaper reported.

Mexican drug cartels have bored scores of tunnels under the US-Mexico border in recent years to circumvent ramped-up security at ports of entry.

In early 2006, agents discovered a tunnel measuring 731 metres running under the same stretch of 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 there seized more than 100 tonnes of marijuana valued at more than $340 million, Mexico's biggest pot haul to date.