Tariffs on imports from Canada and Mexico would go into effect on March 4 “as scheduled,” President Trump said on Thursday morning, claiming that those countries were still not doing enough to stop the flow of drugs into the United States.

China will also face an additional 10 percent tariff next week, on top of the 10 percent he imposed earlier this month, the president wrote in a post on Truth Social.

“Drugs are still pouring into our Country from Mexico and Canada at very high and unacceptable levels,” he said. “A large percentage of these Drugs, much of them in the form of Fentanyl, are made in, and supplied by, China.” He added that the levies were necessary until the flow of drugs “stops, or is seriously limited.”

In an effort to stem the flow of migrants and drugs, Mr. Trump threatened to impose tariffs on all products from Canada, Mexico and China in early February. But after Mexico and Canada promised measures like sending more troops to the border and, in the case of Canada, appointing a “fentanyl czar,” Mr. Trump paused their tariffs for one month.

He moved ahead with imposing a 10 percent tariff on all products from China, on top of those already in place, which prompted China to retaliate with its own tariffs on American goods.

Additional tariffs on the country’s three biggest trading partners would only add to the economic strain that has begun to emerge from Mr. Trump’s flurry of actions. And his threats have posed a particular quandary for Canadian officials, who argue that fentanyl made in Canada has not posed an increasing threat to the United States.