Toyota Motor Corp. (TM) said on Monday it is looking into complaints in the United States that its popular Prius hybrid car (search) suddenly stalled or shut down, often at high speeds on highways.

"We have been alerted to the fact that some owners might have a problem," Toyota (search) spokesman John Hansen told Reuters. "We are going to go back to our service records to find out what owners have had a problem like this and how many."

The U.S. National Highway Traffic Safety Administration (search) said it has received 13 complaints so far of the gas-electric hybrid vehicles stalling or stopping unexpectedly.

Toyota dealers attributed the problems to a software glitch in the car's complex computer system, according to some complaints.

NHTSA spokesman Rae Tyson said the agency is monitoring the complaints but that no formal investigation has been launched.

Toyota has sold 34,225 Prius cars so far this year. There is usually a waiting list to buy the vehicle, which has a gasoline engine with an electric motor to increase fuel efficiency. The Prius gets as much 60 miles per gallon of gasoline.