WASHINGTON (AFP) - United States President Barack Obama offered the United States' condolences after a German airliner crashed Tuesday in the French Alps en route from Spain, calling it "heartbreaking".

All 150 passengers and crew died when the Germanwings airliner came down in a remote area - victims included 16 German teenagers on a school trip.

"It's heartbreaking because it apparently includes the loss of so many children, some of them infants," Mr Obama said, adding that he had called Germany's Chancellor Angela Merkel. "Our teams are in close contact, and we're working to confirm how many Americans may have been on board. Germany and Spain are among our strongest allies in the world."

Mr Obama also called Spanish Prime Minister Mariano Rajoy, offering US assistance. "The president conveyed his condolences and those of the American people to Spain and to the families of those lost on the flight," a White House statement said.