In a letter to US Trade Representative Robert Lighthizer, the senators, led by Republican Senate Finance Committee Chairman Chuck Grassley and ranking Democrat Ron Wyden, said the new trade deal, which replaces the 26-year-old North American Free Trade Agreement, should not enter into force until Mexico and Canada are in full compliance with their commitments.

"Even absent the pandemic, a 1st June deadline would be highly aggressive, and raises questions as to whether businesses have the information they need to adjust to the new rules and comply by that date," the senators wrote. "We ask you to delay the proposed June 1 entry into force and work with Congress and stakeholders to determine a more feasible timeline."

Grassley earlier had raised concerns about the auto industry's ability to quickly comply with higher standards for North American automotive content under the new pact, known as USMCA.

The Mexican Automotive Industry Association on Monday urged its government to seek a delay in the regional automotive rules until 2021.

In a statement to a G20 trade ministers call, Lighthizer on Monday cautioned against using the coronavirus crisis to "push other agendas" in trade and other areas. Instead, the Group of 20 economies should stay focused on defeating the pandemic.

"Let us not make long-term decisions in the midst of a crisis," Lighthizer said.