The United States on Tuesday demanded Toyota hand over documents to officials investigating some of the Japanese automaker's recent vehicle recalls to determine whether they met safety obligations.

The nation's auto safety watchdog, the National Highway Traffic Safety Administration, said it was using its statutory authority to obtain the documents "to determine if the automaker conducted three of its recent recalls in a timely manner" and whether their scope was too limited.

The three recalls for acceleration problems involve Toyota and Lexus vehicles.

The agency, part of the Department of Transportation, said it "is requiring Toyota to provide documents showing when and how it learned of the defects affecting approximately six million vehicles in the US alone."

"Safety recalls are very serious matters and automakers are required to quickly report defects," Transportation Secretary Ray LaHood said in the statement.

The detailed US request for information was the latest blow to embattled Toyota Motor, fighting to restore consumer confidence.

The world's largest automaker has recalled millions of vehicles worldwide in recent months due to problems linked to accelerator and brake functioning that have sullied the company's reputation.

In response to the NHTSA request, Toyota said it took "responsibility to advance vehicle safety seriously and to alert government officials of any safety issue in a timely manner.

"We are reviewing NHTSA's request and will cooperate to provide all the information they have requested."

The number of complaints alleging deaths related to unintended acceleration in Toyota vehicles has surged since Toyota announced on January 26 it was suspending sales and production of eight models in the US.

According to the agency's website Tuesday, 34 deaths allegedly were caused by the problem, including 13 deaths caused by nine accidents between 2005 and 2010 that were reported since late January.

In a related move, with its US sales slumping amid the recalls, Toyota said Tuesday it was suspending production at two plants -- in Kentucky on February 26 and in Texas for two weeks beginning March 15.

The US demand for documents came a day ahead of Toyota president Akio Toyoda's scheduled news conference to provide an update on the progress of massive recalls.

Toyoda, the grandson of the Toyota founder, has publicly apologized at previous news conferences and plans to fly to the United States soon, where the company faces a congressional grilling on February 24 and a host of lawsuits.

Japanese media have said Toyoda is prepared to testify to the US Congress if formally asked.

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

Investigators will probe how the manufacturer learned of the defects in the recalled Toyota and Lexus vehicles, such as via consumer complaints or factory testing, the agency said.

"Investigators are also looking into whether Toyota discovered the problems during pre-production or post-production of the affected vehicles."

Two of the recalls are related to the entrapment of gasoline pedals by floor mats, with the first recall announced on September 26, 2007. That was followed by a recall on October 6, 2009, which was expanded on January 29 to include additional vehicles, NHTSA said.

Toyota announced the third recall, involving sticking gas pedals, on January 21.

The NHTSA said that officials were looking at whether Toyota had covered all affected models in its recent recalls "to ensure Toyota did not miss any problems."

An auto manufacturer found in violation of US safety rules could be liable for a maximum of 16.4 million dollars in civil penalties.

President Barack Obama's transport chief LaHood has vowed "to hold Toyota's feet to the fire" to make sure the company follows through on promises to make their vehicles safe.

Toyota, which in 2008 dethroned US giant General Motors as the world's biggest automaker, has pledged to fix more than eight million vehicles worldwide, more than its entire 2009 global sales of 7.8 million vehicles.