WASHINGTON, D.C. --- Audi is recalling more than 68,000 Audi A6 vehicles because of gas tank problems, according to the National Highway Traffic Safety Administration (NHTSA).

The recall affects 2001-2004 Quattro models with 2.7-, 2.8- or 3.0-liter engines and 2001-2005 Allroad Quattro models with 2.7- or 4.2-liter engines.

The NHTSA said the aging of the plastic material in the fuel tank, combined with bending stress from the aluminum ventilation line to the rollover valve nipple, "may lead to cracking of the nipple, and a fuel leak may result."

In the presence of an ignition source, such a fuel leak poses a fire risk.

Dealers will fix the problem by replacing or reinforcing the nipple on the rollover valve. The recall is set to begin May 30.

Audi's parent company is Volkswagen AG.

For more information about the recall, call Audi at (800) 822-2834.