GENEVA — The World Trade Organization's chief said on Tuesday that US presidential candidate Donald Trump's proposal to impose a 10 percent import tariff would prompt retaliation from partners, resulting in a "lose-lose" situation that could upend the trading system.

"If this is done, it is obviously not helpful to WTO rules. I think it will result most likely in a tit-for-tat approach. Other members will also look to levy these sorts of charges in return; that's what I think," said Ngozi Okonjo-Iweala.

Okonjo-Iweala was responding to a question about the impact of the proposal by Trump, a Republican, to slap a 10 percent tariff on all imports were he to defeat Democratic President Joe Biden in the Nov 5 US election.

"And then I think we will have a little bit of a free-for-all which would upend the stability and predictability of trade," she added, saying she hoped it would not happen.

Okonjo-Iweala, whose campaign to be WTO chief was opposed by the US in 2020 under Trump, was speaking at an event at the Peterson Institute for International Economics in Washington to mark the global trade watchdog's 30th anniversary.

It was the first time she publicly commented on the impact of another Trump presidency on trade.

During his four-year term, Trump regularly criticized the organization's work and blocked appointments of judges to its top appeals bench which remains paralyzed four years later.