Trump will roll out the measures flanked by cabinet members in the Rose Garden of the White House at 4:00 pm (2000 GMT) -- after Wall Street markets close -- promising that they will stop America being "ripped off" and will deliver a new "golden age" of US industry.

But while Trump insisted he had decided on the reciprocal tariffs hitting countries that have targeted the United States, the White House admitted he was still ironing out the details with less than 24 hours to go.

The Republican billionaire has had a long love affair with tariffs, insisting in the face of economic experts that they are a cure-all that will tackle America's trade imbalances with friends and foes alike.

Critics say that not only will US consumers bear the brunt as importers pass on the cost, but that they could increase the risk of a damaging recession at home and abroad.

Global markets have been jittery for days ahead of Trump's announcement, while the countries most likely in the crosshairs have called for talks -- even as they ready retaliatory measures.

- 'Very kind' -

The move also underscores the growing and profound gulf between Trump's America and many of its closest allies, not only on trade but on security, defense and almost everything else.

But in typically unpredictable fashion, the 78-year-old US president is keeping the details close to his chest.

Trump has trailed the measures for weeks, initially suggesting that the tariffs would simply match whatever levies other countries impose.

On Monday, he merely said that he would be "very kind" -- but gave little away.

As the deadline drew near, US media reported that he had also been considering blanket 20 percent tariffs -- and then that he was looking at a third option where some countries would get preferential treatment.

White House Press Secretary Karoline Leavitt said Trump was meeting his top advisors on the eve of the announcement, "perfecting it to make sure this is a perfect deal."

The tariffs would come into effect "immediately" after Wednesday's roll-out, she added -- effectively ruling out any delays for negotiations with other countries.

Trump has wobbled on several other tariff announcements since returning to office in January, blinking at the last minute with allies such as Canada and Mexico.

His plans have however sparked growing fears of a damaging trade war around the world that could drive up prices and cause widespread disruption.

- 'Couldn't care less' -

Major economies including the European Union and Canada have vowed retaliation.

"We are going to be very deliberate in terms of the measures we take, to fight for Canada," Canadian Prime Minister Mark Carney said on Tuesday.

The European Union, which Trump has accused of trying to "screw" the United States, said Tuesday it still hoped to negotiate a solution -- but that "all instruments are on the table" to retaliate if necessary.

British Prime Minister Keir Starmer spoke with Trump on "productive negotiations" towards a UK-US trade deal. Vietnam said on Tuesday it would slash duties on a range of goods to appease Trump.

Meanwhile other top exporters are seeking to build up alliances to counter Washington, with China, Japan and South Korea speeding up talks on a free trade agreement.

Mexico lowered its 2025 growth forecast from between two and three percent to a range of 1.5 to 2.3 percent on Tuesday, citing tensions with its largest trading partner.

Speculation about what Trump has in store has been stoking uncertainty on trading floors.

And HSBC strategists led by Max Kettner warned Wednesday's announcement might not end the tariff uncertainty.

"We'd argue the potential is in fact higher for the 2 April deadline to introduce even more uncertainty -- and hence prolonged broad-based weakness in leading indicators," they said.

Trump has used tariffs as a foreign policy weapon ever since his chaotic first term from 2017-2021.

The former property tycoon insists the levies will bring a "rebirth" of America's hollowed-out manufacturing capacity, and says companies can avoid tariffs by moving to the United States.

Sweeping auto tariffs of 25 percent are also due to come into effect on April 3.

A 25 percent tariff on steel and aluminum from around the world came into effect in mid-March.

China was hit in March by additional 20 percent tariffs on all goods, triggering retaliatory duties from Beijing. The EU has unveiled its own measures to start mid-April.