The United States will place a lower-than-promised 20 per cent tariff on many Vietnamese exports, Donald Trump said on Wednesday (Jul 2), cooling tensions with its tenth-biggest trading partner days before the US president could raise levies on most imports.

Vietnamese goods would face a 20 per cent tariff and trans-shipments from third countries through Vietnam will face a 40 per cent levy, he said. Vietnam could import US products with a zero per cent tariff, he added.

"It is my Great Honor to announce that I have just made a Trade Deal with the Socialist Republic of Vietnam," Trump said on Truth Social after speaking with Vietnam's top leader, To Lam.

Trump's announcement comes just days before a Jul 9 deadline before he ramps up tariffs on most imports, one of the Republican's signature economic policies.

Under that plan, announced in April, US importers of Vietnamese goods would have had to pay a 46 per cent tariff.

Details were scant. It was not clear which products Trump's 20 per cent tariff would apply to, or whether some would qualify for lower or higher total duties.

Also left to later discussion was how the new trans-shipment provision, aimed at products largely made in China and then labelled "Made in Vietnam", would be implemented and enforced.

The Vietnamese government did not confirm the specific tariff levels in a statement celebrating what it described as an agreement on a joint statement about a trade framework.

Vietnam would commit to "providing preferential market access for US goods, including large-engine cars," the government in Hanoi said.

A deal between the two countries would be a political boost for Trump, whose team has struggled to quickly close deals with Washington's biggest trading partners ahead of the deadline.

While the administration has teased a forthcoming deal with India, truces reached earlier with Britain and China were limited in scope. Talks with Japan, the US' sixth-largest trading partner and closest ally in Asia, appeared to hit roadblocks.

The US is Vietnam's largest export market and the two countries' growing economic, diplomatic and military ties are a hedge against Washington's biggest strategic rival, China. Vietnam has worked to retain close relations with both superpowers.

Shares of major US apparel and sportswear makers including Nike, Under Armour and North Face maker VF Corp closed higher on Wednesday on the news.

Lam also asked Trump for the US to recognise Vietnam as a market economy and remove restrictions on the exports of high-tech products to the country, Vietnam said. Those changes have long been sought by Hanoi.

The White House and the Vietnamese trade ministry did not respond to requests for additional comment.