U.S. President Donald Trump on Wednesday accused Iran of delaying peace negotiations and warned that the country would "have to pay the price," while the UN chief urged caution to prevent a return to full-scale war.

The remarks came after Iran and the US once again traded fire following the downing of an American helicopter, further straining a ceasefire that took effect in April but has been marked by sporadic flare-ups of violence, News.Az reports, citing France24.

The exchange drew international calls for restraint on the eve of the World Cup, which the US is co-hosting and Iran is participating in.

Trump had said on Tuesday that talks to bring about a definitive end to the Middle East war were in the "final throes", only to offer a starkly different assessment a day later.

"Iran is all talk and no action," he said on Wednesday. "They've taken too long to negotiate a deal that would have been great for them, now they will have to pay the price!!!"

Separately, Trump told a Fox News journalist that because peace talks had stalled, he was getting closer to targeting Iran's power plants and bridges.

"I may keep going," he was quoted as saying. "They had a chance to sign a deal and survive."

In a sign that diplomacy was continuing, however, negotiators from Qatar -- which along with Pakistan has been assisting in mediation efforts -- travelled to Tehran on Wednesday "to meet with the Iranians in an effort to bridge the remaining gaps", a diplomat with knowledge of the situation said.

At a Security Council meeting on the Middle East, United Nations chief Antonio Guterres suggested that an imperfect ceasefire was preferable to a return to full-scale hostilities.

"We should not minimise the risks of a lesser fire becoming full fire, or in another word -- full war," he said.

News.Az