A list of the passengers given by local media showed five shared the same last name, suggesting they were related.
"The government of Costa Rica profoundly regrets the deaths of 10 US passengers and two Costa Rican pilots in the air accident," which occurred in the country's northwestern Guanacaste region, Costa Rican President Luis Guillermo Solis said in a statement posted to social media.
A US State Department official said: "We confirm the deaths of multiple US citizens in an airplane crash in Costa Rica."
The official said "we express condolences to all those affected by this tragedy," as well as assistance to "affected US citizens, but that "out of respect for the families, we are not sharing additional information at this time."
Costa Rican officials said the plane that came down belonged to Nature Air, a domestic airline that services routes across the country and which also provides aircraft for charter.
Guanacaste is a popular vacation destination, especially this time of year, when its pristine beaches and jungle provide a respite for tourists fleeing the northern hemisphere's winter.
Americans are by far the biggest group of tourists visiting Costa Rica.
The plane that crashed was a single-engine turboprop Cessna 208 Caravan. Its debris was scattered across a wooded mountain close to the beach town of Punta Islita.
Images posted by the country's public security ministry showed blackened wreckage, some in flames, with only part of the tail recognizable.
The cause of the crash was not known.
Solis said in his statement that his government "gives its commitment to do everything necessary to work with the relatives of the victims to give them what they need in this difficult moment."
Early Monday "the investigations into the causes of the accident will begin," Solis said.
Pilot was ex-president's cousin
The Costa Rican newspaper La Nacion gave the names of the 10 passengers as: Thibault Astruc, Amanda Geissler, Charles Palmer, Bruce Steinberg, Irene Steinberg, Matthew Steinberg, William Steinberg, Zachary Steinberg, Leslie Weiss and Sherry Wuu.
No ages were given.
The newspaper reported the plane had taken off from the capital San Jose at 11:37 am (1737 GMT) for the small sealed airstrip in Punta Islita, where the tourists boarded.
The passengers had paid $2,300 for their flight, it said.
The two pilots were identified as Juan Manuel Retana Chinchilla and Emma Ramos.
Former Costa Rican president Laura Chincilla -- Solis' predecessor -- said on Twitter that Retana, 52, was her cousin.
"You will remain in our hearts, dear Juan Manuel Retana," she wrote.
This time of year is particularly busy in Costa Rica's coastal resort areas as tourists and locals alike spend end-of-year holidays in the sun.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)