US President Donald Trump inked plans on Thursday for sweeping "reciprocal tariffs" that could hit both allies and competitors, in a dramatic escalation of an international trade war.

Speaking in the Oval Office, Trump said he had decided to impose reciprocal duties, telling reporters that US allies were often "worse than our enemies" on trade.

"I’ve decided for purposes of fairness that I will charge a reciprocal tariff," Trump said at the proclamation signing. “It’s fair to all. No other country can complain.

"Whatever countries charge the United States of America, we will charge them," Trump added.

In particular, he called the European Union "absolutely brutal" in trade ties with Washington.

With the memo Trump signed on Thursday, officials - including the US trade representative and commerce secretary - will propose remedies on a country-by-country basis.

Trump's commerce secretary nominee Howard Lutnick said that studies should be completed by April 1, and the president could start tariffs as early as April 2.

Besides the EU, the White House also flagged differences in certain US tariff levels with India and Brazil, while noting Japan's "high structural barriers."

Economists caution that sweeping tariffs on US imports would likely boost inflation, not reduce it, in the near term and could weigh on growth eventually.

Trump acknowledged on Thursday that US prices "could go up" due to tariffs, but he expressed confidence that they would ultimately ease. (Agencies)