TD Ameritrade Holding Corp , the big U.S. online brokerage, said on Thursday some of its clients were unable to log on to their trading accounts.

A spokeswoman said the company was unsure of the scope of the problem, which started recently, and was working to solve it. Locked-out clients needed to turn to telephone trading, she said.

(Reporting by Jonathan Spicer, editing by Gerald E. McCormick)