Mexican trucks could have to wait longer than anticipated to haul freight deep into the United States.

The Bush administration in February announced that it would soon allow 100 Mexican trucking companies to travel beyond the current 20-mile limit for a one-year pilot project.

But a Senate panel on Thursday voted to delay the plan by requiring the administration to publish details about it and giving the public time to comment on it. The action came as part of a supplemental spending bill to pay for wars in Iraq and Afghanistan.

"The administration is rushing to open the border to Mexican-domiciled trucks without assuring their safety and enforcement of the law of the U.S.," said Joan Claybrook, president of Public Citizen. "They can't go rushing forward in opening the border without having explained what their pilot project is."

The Transportation Department said it is committed to moving forward with the program and will work with lawmakers to address their concerns.

"The Mexican trucking demonstration program will bring real benefits and real dollars to the American economy while maintaining all U.S. safety and security standards," the department said in a statement.

Democratic Sens. Byron Dorgan of North Dakota, Dianne Feinstein of California and Patty Murray of Washington sponsored the amendment.

"There still remain very significant safety issues," Dorgan said.

He also criticized the Bush administration for opening the border to Mexican trucks before Mexico opened the border to U.S. trucks. "They were going to implement these plans for Mexican long-haul trucks in a way that was at odds with how the Mexicans were going to treat American truckers," Dorgan said.

The move to delay the administration plan was welcomed by the International Brotherhood of Teamsters.

"This is an important first step in our fight to ensure the safety and security of America's highways," said Teamsters spokeswoman Leigh Strope.

Access to all U.S. highways was promised by 2000 under the 1993 North American Free Trade Agreement, as was access through Mexico for U.S. carriers.

That aspect of NAFTA, however, was stalled by lawsuits and disagreements between the two countries, though Canadian and U.S. trucks travel freely across the northern border.

The Bush pilot project will let Mexican truck companies travel from Mexico throughout the United States and back. No hazardous material shipments will be permitted.