WASHINGTON — Toyota Motor Corp. said yesterday it is extending warranty coverage on some 2004-2006 Sienna minivans to address problems with the rear liftgate.

Toyota said there had been 34 complaints about problems opening the liftgate or keeping it in the open position, with allegations of 14 minor injuries. The warranty for the liftgate will be extended from three years or 36,000 miles to six years with no mileage limitations.

The warranty extension involves 585,000 Siennas with either manual or power liftgates.

The National Highway Traffic Safety Administration opened an investigation into the problem in August 2006. At the time, NHTSA said it had received eight complaints and four reports of injuries involving the liftgate unexpectedly closing on a person's head or body.

Toyota spokesman Bill Kwong said the part is "not defective" but has been "susceptible to degradation." He said the automaker did not consider it to be a safety issue.

Toyota issued a technical service bulletin in March 2004 to owners of the 2004 Sienna saying that the liftgate struts had been redesigned to help prevent seal damage.

Owners will receive a letter informing them of the warranty upgrades. Customers with questions can call Toyota at 800- 331-4331.