US President Donald Trump says the ceasefire agreement with Iran is "over", blasting the country's leadership as "scum" and "cuckoo" after fresh exchanges of fire.

Speaking ahead of a Nato summit in Turkey, the president said US negotiators could continue talks "if they want" but said he saw it as "a waste of time".

Oil prices spiked by around 6% after Trump's comments, although are still well below the highs seen during the full closure of the Strait of Hormuz.

The US and Iran renewed fighting late on Tuesday into Wednesday, in the worst exchange of fire since they signed an interim deal in June that called for an end to the conflict and safe passage of vessels through the strait.

"I think it's over. I don't want to deal with them anymore they're scum ... they're led by sick people and they're vicious, violent people," Trump said.

"We make a deal. They [Iran] go outside, talk to the press, they say 'we never even talked about it'. There's something wrong with them. They're cuckoo. As far as I'm concerned, it's over."

The fresh wave of strikes began after three oil tankers were attacked in the Strait of Hormuz, with US Central Command (Centcom) saying on Tuesday it had launched "powerful" strikes in response.

On Wednesday, Iran said it targeted US military sites in Bahrain and Kuwait in retaliation to the US strikes.

The US also said it had revoked its temporary suspension of sanctions on Iranian oil sales.

Iran's parliamentary speaker Mohammad Bagher Ghalibaf accused the US of breaching their Memorandum of Understanding (MoU) on this issue, and others, including the attacks in southern Iran and "violating Iranian adjustments in the Strait".

"The era of bullying and extortion is over. It leads nowhere. We don't fold," he said.

It is not the first time strikes occurred after the MoU was signed on 17 June.

The US launched a series of strikes on Iran on 25 June after an Iranian projectile hit a cargo ship in the Strait of Hormuz. By 29 June both sides agreed to "stand down".

Part of the MoU's 14 points is for an "immediate and permanent termination of military operations on all fronts".

Iran agreed to use its "best efforts for the safe passage of commercial vessels with no charge for 60 days".

Both sides had continued negotiations on the terms of a permanent end to the war, but talks were paused during funeral ceremonies for the late Ayatollah Ali Khamenei, who was killed on the first day of US-Israel strikes on Iran.

Ceremonies are taking place in Iraq on Wednesday, with the final rites and burial set for Mashhad in north-east Iran on Thursday.

It is not clear when talks will resume after this latest round of strikes, as Trump responded to a question about whether talks would resume after the funeral with: "I don't care".

"Frankly, I don't want to waste my time with them. Now, I'll let our wonderful negotiators keep talking if they want, but I don't see it," Trump said.

"As far as I'm concerned, it's just a waste of time dealing with them. They're liars."