Transport Minister Marc Garneau said today the federal government isn't ruling out delaying Canada's ratification of the new North American free trade pact until after the U.S. lifts controversial tariffs on steel and aluminum imports.

Garneau, who chairs the cabinet committee on Canada-U.S. relations, said holding back on ratifying the Canada-United-States-Mexico Agreement (CUSMA) would create challenges, but it's something his government is considering.

The government tabled the text of the treaty in mid December; it can't introduce implementation legislation before March.

"We will, as you know, look at the implementation after the 18th of March, but what I'm saying to you is that we will be doing some serious thinking about whether we want to proceed forward with it ... you know the situation with respect to steel and aluminum is not yet resolved," he told CBC Radio's The House from Washington, D.C. Thursday. The full interview will air Saturday.

"I'm not saying that it's a showstopper, but it certainly is an impediment because we have a good deal with the United States and — without the tariffs in place — we're very happy with the deal."

Meanwhile, Canada's top man in Washington said today he thinks U.S. tariffs on Canadian steel and aluminum will be lifted in the next few weeks — but wouldn't elaborate on what's motivating that optimism.

David MacNaughton, Canada's ambassador to the U.S., would only tell reporters at a gathering in Washington that there is a lot of support in the U.S. among workers and politicians for killing the punitive tariffs.

"We'll get there in the next few weeks," he said. "We're getting there."

Last May, the U.S. announced tariffs of 25 per cent on imported steel and 10 per cent on imported aluminum.

Canada retaliated with tariffs of its own, but the government has faced criticism for signing a new North American trade pact with the U.S. and Mexico without securing any guarantees from Washington that it would lift the tariffs.

MacNaughton said he's met with United States Trade Representative Robert Lighthizer and members of Congress over the last few weeks.

"I'm optimistic that we are going to get there," he said.