President Donald Trump promised a tax on imports, reviving an idea he floated earlier in his administration that risks escalating tensions with key U.S. trade partners.

“We’re going to be doing very much a reciprocal tax,” Trump said Monday at a White House event on infrastructure. “And you’ll be hearing about that during the week and during the coming months.”

Trump didn’t elaborate on his plan at the event. Later in the day, a senior administration official said there is no formal proposal for such a tax in the works and the president was simply reiterating sentiments he has long held.

Trump shifted during the discussion of infrastructure to blast “very unfair” treatment of the U.S. by “so-called allies, but they’re not allies on trade” that charge tariffs on U.S. producers yet benefit from lower U.S. trade barriers to export to America.

“We cannot continue to be taken advantage of by other countries,” Trump said in the White House meeting with state and local officials. “We cannot continue to let people come into our country and rob us blind and charge us tremendous tariffs and taxes and we charge them nothing. We cannot allow that to happen.”

Trump emphasized the point in an exchange later during the event with Commerce Secretary Wilbur Ross, saying “oh, would he be in trouble” if he didn’t support the tax. “Could you imagine if he said no,” Trump said to laughter.

Ross followed with a full-throated endorsement of such a measure. “We gave away so much unilaterally that we really have to claw it back” from trading partners, he said.

Trump raised the idea early last year, as the administration explored plans for its tax overhaul. Economists and tax experts parsed Trump’s prior remarks at the time to mean he was calling for import tariffs—that is, taxes levied on specific goods or countries at varying rates. In describing his vision, Trump called for taxing imports from other countries at the same rates those countries impose on U.S. products.

A House Republican proposal to tax imports, known as the border-adjusted tax, was removed from tax revamp plans after facing intense opposition from import-heavy industries such as retailers, and a cool reception from Senate lawmakers.

“When you say I’m going to charge a 10 percent or a 20 percent border tax, everyone goes crazy, because they like free trade.” Trump said during an interview aired by Fox Business Network in April. He added later: “But when you say ‘reciprocal tax,’ nobody can get angry.”

“You say, ‘OK, whatever you charge, we’re charging,”’ Trump added in the interview.