US transportation safety officials have said they are investigating complaints from some Toyota owners that their vehicles are still accelerating unexpectedly, even after being repaired under the company's recent recall programme.

Toyota has recalled more than eight million cars for the fault, which it maintains can be solved by a small fix to the accelerator pedal assembly, or the floor mat underneath it.

The company has also included a software upgrade designed to ensure the brakes override the accelerator.

But officials at the US Department of Transportation say at least 15 people have reported subsequent problems.

In a statement on Wednesday David Strickland, administrator of the National Highway Traffic Safety Administration, said the agency wanted to hear from any other Toyota owners experiencing problems.

His department, he said, wanted "to get to the bottom of the problem and to make sure Toyota is doing everything possible to make its vehicles safe."

"If Toyota owners are still experiencing sudden acceleration incidents after taking their cars to the dealership, we want to know about it," Strickland said.

Speaking to the Associated Press Toyota owner Carolyn Kimbrell recalled how days after receiving a fix to the accelerator pedal her 2006 Avalon suddenly sped up outside her home.

"It just scares you," she said. "If I had been trying to stop at a busy intersection, that would have been bad."

The fresh complaints raise new questions about whether Toyota's previously announced fixes will solve the problem.

The US National Highway Traffic Safety Agency has said it is investigating more than 50 deaths linked to crashes allegedly caused by Toyota's acceleration problems.

Toyota, which says it has so far fixed about 1 million recalled cars, said on Wednesday it was looking into the new complaints.

The recalls have badly dented Toyota's reputation for safety and quality, prompted three separate investigations in the US Congress and generated several class action death and injury lawsuits.

Meanwhile federal prosecutors in New York are conducting a criminal investigation into the recalls and the Securities and Exchange Commission is probing what the
carmaker told investors.

Responding to the latest complaints from Toyota owners, Bruce Braley, a Democratic congressman who serves on one of the committees investigating Toyota said he was "deeply concerned" at the number of new incidents affecting cars that were supposed to have been fixed.

"It's critical that we get to the bottom of this problem as quickly as possible," he said.