At least eight people were killed when two suspected smuggling boats capsized off the coast of southern California, rescuers said Sunday.

"We lost eight souls," the lifeguard chief for the city of San Diego, James Gartland, told a press conference.

"This is one of the worst maritime smuggling tragedies that I can think of in California, certainly here in the city of San Diego," he added, in apparent reference to clandestine migration into the United States.

It was unclear exactly what caused the nighttime accident in an area Mr Gartland described as "hazardous," due to sand bars and in-shore rip currents.

He said a Spanish-speaker had called the 911 emergency number just before midnight on Saturday, saying there were some 23 people between two small, open boats -- eight on one vessel and 15 on another -- that had overturned at Torrey Pines beach in San Diego, a city near the Mexican border.

Rescuers did not find any survivors, but some "may have left the beach" by the time the teams arrived on scene, Mr Gartland said.

All of the them were adults, he added, saying he had no further details on their nationalities.

Large numbers of migrants from South and Central America cross US borders clandestinely, often taking enormous risks in the hopes of reaching the United States.