A US federal court has blocked President Donald Trump's sweeping tariffs, in a major blow to a key part of his economic policies.

The Court of International Trade ruled that an emergency law invoked by the White House does not give the president unilateral authority to impose tariffs on nearly every country.

The Manhattan-based court said the US Constitution gives Congress exclusive powers to regulate commerce with other nations, and this is not superseded by the president's remit to safeguard the economy.

Within minutes of the ruling, the Trump administration lodged an appeal.

The court also blocked a separate set of levies the Trump administration imposed on China, Mexico and Canada since returning to the White House, in response to what it said was the unacceptable flow of drugs and illegal immigrants into the US.

"It is not for unelected judges to decide how to properly address a national emergency," White House deputy press secretary Kush Desai said in a statement.

"President Trump pledged to put America First, and the Administration is committed to using every lever of executive power to address this crisis and restore American Greatness," he added.

The lawsuit, filed by the nonpartisan Liberty Justice Centre on behalf of five small businesses that import goods from countries targeted by the duties, was the first major legal challenge to Trump's so-called "Liberation Day" tariffs.

The attorney general of New York, one of 12 states involved in the lawsuit, welcomed the decision.

"The law is clear: no president has the power to single-handedly raise taxes whenever they like," Letitia James said.

"These tariffs are a massive tax hike on working families and American businesses that would have led to more inflation, economic damage to businesses of all sizes, and job losses across the country if allowed to continue," she added.

The case is one of seven legal challenges to the administration's trade policies, along with challenges from 13 US states and other groups of small businesses.

In the ruling, a three-judge panel said the International Emergency Economic Powers Act (IEEPA), a 1977 law that Trump cited to justify the tariffs, does not give him the power to impose sweeping them.

"The Worldwide and Retaliatory Tariff Orders exceed any authority granted to the President by IEEPA to regulate importation by means of tariffs. The Trafficking Tariffs fail because they do not deal with the threats set forth in those orders," they wrote.

Global financial markets have been on a rollercoaster ride since Trump announced the sweeping tariffs on 2 April, as some measures were reversed or reduced as the White House negotiated with foreign governments.

Stock markets rose in Asia on Thursday morning, with Japan's Nikkei 225 index up by around 1.5% and the ASX 200 in Australia up a little.

US stock futures also jumped after the court ruling. Futures are contracts to buy or sell an underlying asset at a future date and are an indication of how markets will trade when they open.

The US dollar also made gains against safe-haven peers, including the Japanese yen and Swiss franc.