Washington DC - President Donald Trump warned on Wednesday that he would renew US attacks on Iran, saying Tehran had taken too long to agree a deal to end the Middle East war and accusing its negotiators of "playing us for suckers."

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.

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, including from the head of the United Nations who cautioned against a return to all-out war.

"We hit them hard yesterday. We're going to hit them again hard today," Trump told reporters in the Oval Office.

Donald Trump Trump urges judge to toss out lawsuit trying to block his UFC fight

"We were really close to a deal, but they keep tapping us along, they keep playing us for suckers."

The US leader had earlier accused Iran of taking too long to negotiate a peace deal after weeks of talks and warned it would "have to pay the price," offering a different assessment from a day before when he said talks to agree a peace deal were in the "final throes."

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

In a sign that diplomacy was continuing, however, negotiators from Qatar – which along with Pakistan has been assisting in mediation efforts – traveled 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 minimize the risks of a lesser fire becoming full fire, or in another word – full war," he said.