A bus plunged off a cliff in a mountainous area north of Costa Rica's capital on Thursday, killing 11 people on board and injuring 12, rescue officials said.

The passengers were retired staff members of Costa Rica's National University on their way to an indigenous reserve to carry out social work, the rector of the institution said.

"It looks like the bus's brakes failed," Esteban Gonzalez, the son of one of the survivors, told a Costa Rican television network.