Four people died and some two dozen people were injured after a boat got into trouble off the coast of San Diego in a suspected smuggling operation.

The overcrowded 40-foot (12m) cabin cruiser broke up on a reef near Point Loma on Sunday morning, tipping some 30 people into the water, officials said.

A major rescue operation was launched to help the injured on shore and retrieve seven people from the sea.

Officials said the boat captain was in custody on suspicion of smuggling.

"Every indication from our perspective is that this was a smuggling vessel used to smuggle migrants into the United States illegally," Jeff Stephenson, Supervisory Border Patrol Agent, said.

"We haven't confirmed the nationality of the people involved, but our agents are with many of them at the hospital and the man who we believe was the operator... The investigation's still unfolding."

Rescuers were alerted to the incident near the Cabrillo National Monument, at about 10:30 local time (17:30 GMT).

Rick Romero, of the San Diego Fire Rescue Lifeguard, said first reports indicated three or four people needed help but they quickly realised it was "going to be a bigger situation with more people".

"Once we arrived on scene, the boat had basically been broken apart," he said. "Conditions were pretty rough: five to six feet of surf, windy, cold."

He said people were in the water being taken out to sea by the rip current, while people on shore needed CPR and treatment for hypothermia and other injuries.

In a statement, the US Coast Guard said 29 people had been accounted for, of which 24 people were alive, four people had died and one was in a critical condition.

Footage from the scene showed large pieces of debris washing up on the beach. Jet skis, three more rescue boats and specialist rescue equipment including cranes were brought in to help with the rescue.

At the same news conference, Mr Stephenson said it was not yet known where the boat had come from but many smuggling boats come from Mexico's Baja coastline.

"The smugglers, they don't care about the people they're exploiting. All they care about is profit to them," he said. "These people are just commodities. So you can see that in the way they treat them, inadequate safety equipment, really poorly equipped vessels and giving them minimal."

Mr Stephenson said there had been a 92% increase in the number of apprehensions linked to smuggling by sea in 2020, compared with the previous year.

US Customs and Border Protection (CBP) had announced on Friday that it was beefing up its patrols of the coastline over the weekend to try and deter smugglers.

Border patrol agents detained 21 people - 15 men and six women - found on a small open boat, a panga, off the coast of San Diego early on Thursday. They were Mexican nationals with no legal status to enter the US, the CBP said. Two suspected smugglers who were on board the boat will face federal charges.