MIAMI (AFP) - US Coast Guard authorities searched waters off Puerto Rico on Friday (Jan 28) after a boat carrying more than 20 people capsized in the second migrant smuggling accident in a week.

The Coast Guard said it received a call around 1am (1pm Singapore time) reporting people in the water off the north-west coast of the US territory in the Caribbean.

They had been aboard a makeshift vessel carrying out a suspected human trafficking trip to Puerto Rico, the Coast Guard said in a statement.

The boat flipped over about 70 metres off a beach in the town of Isabela. The caller said multiple people were seen running inland from the water.

Authorities in Puerto Rico detained 12 of those migrants, whose nationality has yet to be determined.

Survivors said there were 27 people on the boat, the Coast Guard statement said.

"Border Patrol authorities continue to work to determine how many people were actually on the vessel and how many others made it safely to shore," it added.

A rescue helicopter is looking for possible survivors in the area of the accident.

On Thursday, the Coast Guard called off a search for 34 people missing from a boat that capsized off Florida after hitting rough water on a suspected human smuggling trip from the Bahamas.