The United States is "disappointed" Mexico has decided to impose retaliatory duties on additional U.S. goods in a dispute over cross-border trucking, the top U.S. trade official said.

"Mexico is an important U.S. export market and President (Barack) Obama understands the economic pain that these tariffs cause for American farmers, companies and workers," U.S. Trade Representative Ron Kirk said in a statement.

"We are committed to continuing to work with members of Congress and our counterparts in Mexico to resolve the dispute and end these duties," he said.

Mexican officials said they were imposing additional tariffs because of the slow U.S. response in resolving the dispute, which stems from a commitment the United States made in the North American Free Trade Agreement to open its market to Mexican trucks.

In 2009, Mexico slapped retaliatory duties on 89 U.S. goods accounting for about $2.4 billion worth of trade, after the U.S. Congress canceled a pilot program for long-haul Mexican truckers started by former President George W. Bush.

The new retaliation list has been expanded to include 99 products, including pork, a spokesman for Mexico's embassy in Washington said. It will be published in coming days, he said.

Kirk said he and Transportation Secretary Ray LaHood have been working other agencies and members of Congress to resolve the dispute a way that upholds U.S. trade obligations, as well as safety concerns raised by opponents of opening the U.S. market to the Mexican truckers.

LaHood told Senator Patty Murray on May 6 the Obama administration would be announcing a proposal to fix the dispute "very soon."

Murray's home state of Washington, which Obama visits, has been hit hard by the tariffs. (Reuters)