WASHINGTON — President Donald Trump on Friday said the United States would terminate all trade discussions with Canada, “effective immediately,” over the country’s plan to begin collecting digital services taxes from U.S. technology giants.

Trump described those taxes as a “blatant attack,” and promised on social media that he would inform Canada within the next seven days about the duties that “they will be paying to do business with the United States of America.” In ceasing talks, Trump once again upended the increasingly fraught relationship between the United States and Canada, which has traditionally been one of America’s closest allies and largest trading partners.

Those relations had seemed to be improving after Canada’s new prime minister, Mark Carney, took office. The two nations even appeared to be on track to announce a trade deal in July, having huddled at length this month on the sidelines of the Group of 7 summit, which was hosted in Alberta.

But on Friday, Trump signaled that Canada was back in the penalty box.

“We have all the cards. We have every single one,” Trump said in the Oval Office, adding that “economically, we have such power over Canada.”

Canadian officials did not respond to requests for comment on whether Carney would try to call Trump or step back from collecting the digital tax. But Carney’s office said in a statement that his government would “continue to engage in these complex negotiations with the United States in the best interests of Canadian workers and businesses.”

A decision by Trump to follow through and increase tariffs on a North American neighbor could unleash chaos on both sides of the border, with U.S. companies and consumers facing the brunt of higher import costs. Financial markets briefly recoiled over the breakdown in negotiations, although the S&P 500 recovered and reached a new high Friday.