The timetable for renegotiating the North American Free Trade Agreement is slipping, and Canada has no idea when the U.S. administration will start the process, a senior Canadian official said on Thursday.

President Donald Trump says NAFTA has been a disaster for U.S. workers and is threatening to withdraw from the pact unless major changes are made. He must give Congress 90 days' notice under trade law before beginning formal talks.

"Obviously the 90-day notice period has not been given yet so it (giving notice) is slipping somewhat. I have no idea when it will happen," said David MacNaughton, Canada's ambassador to the United States, and a key player on the file.

"I keep being assured it's imminent but imminent seems to be dragging on ... we can't control the U.S. process," he told reporters after a private meeting of cabinet ministers and senior officials to discuss the talks.

U.S. Commerce Secretary Wilbur Ross said on March 30 he hoped to start the 90-day countdown by April 7.

Trump though does not have a chief trade negotiator in place. In Washington, the Senate Finance Committee on Thursday delayed a scheduled vote on the president's nominee for the job and members will not be able to address the matter again until April 24 at the earliest.

Canadian officials say privately they have little idea what the administration intends to do about NAFTA.

Although Trump said in February he only wanted to tweak U.S.-Canadian trade, some ideas circulating among policy makers in Washington could badly harm Canada's economy, such as a border tariff and a Buy American policy.

MacNaughton said he did not think a border tax would be imposed, on the grounds that such a measure would cut U.S. jobs and hike the prices for products.

Former Conservative Prime Minister Brian Mulroney, a driving force behind the 1994 trilateral deal with the United States and Mexico, said the talks would be tough, especially when it came to rules of origin and mechanisms to settle investor disputes.

"It will be extremely difficult," he said after addressing the private meeting.

Asked whether the likely changes would amount to more than Trump's promised tweak, he replied: "I think so."

Liberal Prime Minister Justin Trudeau has turned to Mulroney to help smooth ties with Trump and protect exports.