WASHINGTON (Reuters) - The Bush administration on Monday extended a test program allowing long-haul trucks from Mexico full access to U.S. highways for up to two years, despite pending legislation in Congress to shut it down.

'We intend this extension to reassure trucking companies that they will have sufficient time to realize a return on their investment, and we anticipate additional participation with this extra time,' said John Hill, the Transportation Department's top trucking safety regulator.

Participation has been limited, regulators said, because of political wrangling about the program's future.

Organized labor, highway safety and consumer groups have fiercely opposed the initiative, which was permitted under NAFTA -- the North American Free Trade Agreement.

Trucks from Mexico have historically been confined to U.S. border areas where they offload goods to be trucked by American companies.

Last year, regulators approved a one-year pilot program to allow a limited number of Mexican trucks full access to U.S. roads over congressional objections. American trucks were also allowed to operate in Mexico.

Ten U.S. carriers with 55 trucks and 27 Mexican carriers with 107 trucks have participated in the program as of July without incident, U.S. and Mexican officials said.

They also said vehicles participating in the program meet safety requirements. But officials from U.S.-based unions say disputed safety issues remain unresolved. ....