The US National Highway Traffic Safety Administration (NHTSA) is using statutory authority to obtain documents from Toyota to determine if the automaker conducted three of its recent recalls in a timely manner, it said.

US federal law requires all auto manufacturers to notify NHTSA within five days of determining that a safety defect exists and promptly conduct a recall.

"Safety recalls are very serious matters and automakers are required to quickly report defects," said US transportation secretary Ray LaHood in a statement.

The auto safety agency has demanded Toyota provide documents showing when and how it learned of the defects affecting approximately 6m vehicles in the US alone. The probe will examine how the manufacturer learned of these defects, such as through consumer complaints or factory testing. Investigators are also looking into whether Toyota discovered the problems during pre-production or post-production of the affected vehicles.

Officials are checking whether Toyota has covered all affected models in its recent recalls to ensure the automaker did not miss any problems. The agency will obtain information on production data, incidents, complaints, warranty complaints, copies of tests, dates of meetings, timelines, and supplier information.

The three recalls in question involve various Toyota and Lexus vehicles. Two of the recalls are related to the entrapment of gas pedals by floor mats. The first recall was announced on 26 September, 2007 and was followed by a subsequent one on 6 October, 2009. The October recall was expanded on 29 January, 2010, to include additional vehicles. The third recall, involving sticking gas pedals, was announced on 21 January, 2010.

"Our top priority is safety and we expect that all manufacturers address automotive safety issues quickly and in a forthright manner," said NHTSA administrator David Strickland.

NHTSA noted it had authority to seek civil penalties for a variety of violations by manufacturers, equipment suppliers, registered importers and vehicle customisers. If agency officials determine that an automaker violated its statutory obligations, it could be liable for a maximum of US$16.4m in civil penalties.

Toyota's is also recalling about 437,000 units of four hybrid models globally, including the latest version of the Prius to update brake software after owners complained about the way the system integrates regenerative and hydraulic braking on some road surfaces.

It has also recently announced a recall of 8,000 2010 Tacoma trucks in the US and is investigating a possible power steering fault in a small number of Corolla models.

In Japan, the automaker today announced a raft of new measures to boost quality.