The measure approved last week by a Senate committee contains tougher safety requirements for U.S.-bound Mexican trucks than those proposed by the administration.

To comply with the 8-year-old North American Free Trade Agreement, Bush wants Mexican trucks to be allowed on roads throughout the country beginning next January. The treaty required Mexican trucks have access to American roadways by January 2000.

Currently they are restricted to a commercial zone along the border.

The administration said the Senate committee's bill would violate NAFTA.

``Unless changes are made to the Senate bill, the president's senior advisors will recommend that the president veto this bill,'' the White House said in a statement.

The White House last week also said that Bush would veto the House version of the transportation appropriations bill. That legislation blocks the transportation department from spending money to process applications for Mexican trucks and contains no money for enforcement of safety requirements.

The Senate committee's bill, drafted by Sens. Patty Murray, D-Wash., and Richard Shelby, R-Ala., was expected to go to the Senate floor this week and had backing from senators from both parties. It includes $100 million -- $15 million more than the White House requested -- for safety measures.

The administration said although the measure was an improvement from the House version, it had concerns about the safety requirements.

Murray said assertions that the measure violates NAFTA are ``utterly untrue.''

``Despite its veto threat, the administration has yet to address its inadequate response to ensuring the safety of our highways,'' Murray said.

Sens. John McCain, R-Ariz, and Phil Gramm, R-Texas, are drafting an amendment to soften some of the provisions in the Murray-Shelby bill. McCain said some of its requirements would delay the entry of Mexican trucks by two to three years.

Gramm and McCain said they would try to resolve differences in the bill before it gets to the Senate floor. They said the bill likely would not be considered until next week.

``I do believe that again it's inappropriate to have a major provision which, in the view of the administration, violates NAFTA, which would provoke a presidential veto on an appropriations bill,'' McCain said.

Sen. Kay Bailey Hutchison, R-Texas, expressed optimism that an agreement could be reached, but was steadfast in insisting on stronger safety measures. Hutchison helped draft the Senate legislation that the administration is threatening to veto.

``I think the department of transportation needs to do more than they've done,'' Hutchison said. ``I think we can achieve that and still keep the commitment to NAFTA, but nothing in NAFTA says that we'll allow unsafe trucks on American highways. I'm not going to accept watered-down language that doesn't achieve the goal of safety.''