Visitors look at Toyota Motor Corp's hybrid cars at the company's showroom in Tokyo Feb 5, 2013.?[Photo/Agencies]

US safety regulators opened an investigation into an estimated 30,000 Toyota Camry hybrid electric sedans after receiving numerous consumer complaints about braking problems.

The US National Highway Traffic Safety Administration said it had begun a preliminary evaluation into the hybrid Camry sedans from model years 2007 and 2008. The agency said it had gotten 59 complaints of intermittent loss of assisted braking, resulting in increased stopping distances.

A preliminary evaluation is the first step in a process that can lead to a recall if regulators determine that a manufacturer needs to address a safety problem.

Two accidents but no injuries were listed in documents filed online with the NHTSA. The complaints show an increasing trend, with more than half received in the past eight months, according to the documents.

Each complaint alleges multiple incidents of either delayed braking activation or increased brake pedal effort, according to the NHTSA documents. About 40 percent of the incidents occurred at speeds of 40 miles per hour or more, according to the NHTSA.

A Toyota spokesman said the company was cooperating with the NHTSA's investigation.