By Marty Graham

SAN DIEGO (Reuters) - U.S. federal agents have arrested four people and seized nearly 3,000 pounds of marijuana in an operation to bust a drug smuggling ring that had tunneled under the U.S.-Mexico border, federal prosecutors said on Wednesday.

The smugglers built a house to hide the northern entrance to the 400-yard (365 meters) tunnel in Calexico, the U.S. Attorney's Office for southern California said.

The southern end of the tunnel came up in El Sarape restaurant in Mexicali, authorities said.

It is the first smuggling tunnel found in more than 10 years in Calexico, a small city of about 40,000 people some 120 miles (193 km) east of San Diego, but the 12th discovered along Mexico's border with California since 2006.

The rest of the tunnels were found in the San Diego area.

The smugglers purchased the land last April and by the time construction wrapped up in December, federal investigators were using wiretaps and watching activity at the house, charging documents said.

"This house and tunnel were constructed under the watchful eye of law enforcement," said Laura Duffy, U.S. Attorney for southern California. "For the builders, the financiers and the operators of these passageways, there is no light at the end of the tunnel."

Federal agents continued to monitor the house, a separate safe house in Calexico and four suspects until the first loads of marijuana came through at the end of February.

On Mar. 7, investigators seized 1,350 pounds of pot that came through the tunnel as it was being transported through the Los Angeles area, according to federal documents.

On Tuesday, a mother and daughter who were allegedly part of the ring were arrested in nearby Nogales, Arizona.

On Wednesday, federal investigators arrested two men in Calexico - one at the tunnel house and one at the safe house - and seized an additional 1,532 pounds of marijuana from the tunnel.

The suspects will be charged with conspiracy to import and distribute marijuana, prosecutors said.

(Reporting by Marty Graham in San Diego; Editing by Curtis Skinner and Simon Cameron-Moore)