U.S. safety regulators opened a formal investigation on Thursday into consumer complaints about braking on 2010 Toyota Motor Corp Prius hybrids.

The Transportation Department's National Highway Traffic Safety Administration (NHTSA) said it has received 124 complaints about momentary braking problems after motorists rolled over bumps or potholes. Four crashes were alleged by motorists to have been caused by the problems.

Toyota officials declined immediate comment. But the automaker's engineers in Japan said they had fixed a software problem related to anti-lock brakes on the new Prius model.

Late on Wednesday, Transportation Secretary Ray LaHood spoke with Toyota President Akio Toyoda, who reassured him that Toyota takes U.S. safety concerns seriously, a Transportation Department statement said.

Toyota shares, which fell 6 percent on the New York Stock Exchange on Wednesday, were off another 1.8 percent to $72.18 in early trading on Thursday. They closed off 35 percent in Tokyo. (Reporting by John Crawley, editing by Gerald E. McCormick)