"It's too soon to meet, I didn't want to," Trump told reporters at the summit, saying he knew that Foreign Minister Mohammad Javad Zarif was going to drop in for unscheduled talks.

"I knew he was coming," Trump said of the visit, which was engineered by France's Emmanuel Macron in a bid to break the diplomatic deadlock over Tehran's disputed nuclear programme.

"I knew everything he (Macron) was doing and I approved everything he was doing," Trump said, adding the French president "asked for my approval".

A landmark 2015 nuclear deal between Western powers and Iran all but collapsed after Trump unilaterally withdrew US support last year, reimposing sanctions that have crippled Iran's economy.

Macron has been trying for months to defuse the tensions between Tehran and Washington by finding ways of resuming dialogue, and held talks with Zarif in Paris on the eve of the summit.

Trump insisted that Washington was interested only in curbing Iran's nuclear ambitions.

"We're not looking for regime change. You see how that works (in other places)," he said. "What we want is very simple... non-nuclear."