One of the border fences that separates San Diego in the US and Tijuana, Mexico, follows hilly terrain [Reuters]

More than 150 secret tunnels for smuggling people and drugs into the US have been found on its long, porous border with Mexico, some in use for more than two decades, US officials said.

"Illicit tunnel activity has been on the rise since the first documented tunnel was discovered in 1990. Since then, 154 tunnel attempts have been discovered, all but one of which were located along the southwest border" with Mexico, US Immigrations and Customs Enforcement (ICE) official James Dinkens told legislators on Wednesday.

"Over the past several years, law enforcement has seen a marked increase in the number and sophistication of tunnels," he told a senate hearing.

California Democratic Senator Dianne Feinstein said at the hearing that she would introduce a bill seeking to tighten an illegal tunnel law passed in 2007. The idea is to make illegal tunnel-making a more serious offense: conspiracy.

Half of the tunnels authorities identified - used by drug cartels and people-smuggling operations - were located between the US state of California and Mexico's Baja California Norte state, Laura Duffy, US Attorney for the Southern District of California, said.

Criminal enterprise

Some of the tunnels are quite rustic and others are well-made with internal rail systems and ventilation, officials said.

"It is a real serious penetration into the US," Feinstein, who leads the senate's anti-drug trafficking committee, said. "They could mobilise large groups."

Only one person, meanwhile, has been prosecuted for "crime tunnel" building, she noted.

The toll in suspected drug-related violence in Mexico has surpassed 37,000 since its President Felipe Calderon launched a military crackdown on organised crime in 2006.

Gun trafficking

US firearms agents also told legislators on Wednesday they were instructed only to watch as hundreds of guns were bought, illegally resold and sent to Mexico where drug-related violence has raged for years.

Agents for the Bureau of Alcohol, Tobacco, Firearms and Explosives in Arizona told the House of Representatives Oversight Committee they were commanded not to arrest the so-called straw buyers and instead see where the guns went.

Republicans and Democrats on the panel expressed outrage about the ATF program - "Operation Fast and Furious" - and demanded answers from the Obama administration about why arrests were secondary to tracking the firearms.

"We monitored as they purchased handguns, AK-47 variants and .50 caliber rifles, almost daily at times," John Dodson, an ATF special agent in Phoenix, told the committee.

"Rather than conduct any enforcement actions, we took notes, we recorded observations, we tracked movements of these individuals, we wrote reports but nothing more."

Dodson said agents were never given reasonable answers why their activities were limited.