Washington (AFP) - President Donald Trump lost his nominee for Pentagon chief on Tuesday, adding to the volatility in a tense standoff with Iran, which claimed to have dismantled a CIA network.

Foreign powers are watching the situation in the Mideast with growing concern as Tehran and Washington exchange warnings about an escalation in their conflict.

Trump -- in his latest comments that appear to contradict more hardline advisors -- told TIME magazine late Monday he considers explosions on two oil tankers, blamed by Washington on Iran, to have been only "very minor."

But his policy was thrown into further uncertainty by the abrupt withdrawal of his pick for defense secretary.

Trump announced on Twitter that Patrick Shanahan was quitting to spend time with his family. The former Boeing executive has seen his candidacy bogged down for months in Congress and the last straw appeared to be revelations of violent incidents during his previous marriage.

Trump named army secretary Mark Esper as his new pick. He will start work on June 24.

The upheaval leaves the Pentagon without a permanent boss since the shock resignation of the respected James Mattis in December last year.

Despite Trump's comments to TIME, Secretary of State Mike Pompeo warned Iran that the United States is serious about curbing what it calls Iranian aggression.

Another 1,000 troops were ordered to deploy on Monday, in addition to powerful air and naval forces sent earlier.

That deployment should "convince the Islamic government of Iran that we are serious and to deter them from further aggression in the region," Pompeo said Tuesday.

Trump also told TIME he would "certainly" order a war if necessary to stop Iran getting nuclear weapons -- something the country says it does not want to do.

- Spy claims -

In Tehran, officials said they had dismantled a US spy network.

Tehran's announcement came a day after it said its uranium stockpile would from June 27 begin to surpass a limit agreed in a 2015 international nuclear deal that Trump unilaterally abandoned in May last year.

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Tensions between Tehran and Washington have grown ever since, with the US bolstering its military presence in the region, reimposing sanctions and blacklisting Iran's elite Revolutionary Guards as a terrorist organization.

"Following clues in the American intelligence services, we recently found the new recruits Americans had hired and dismantled a new network," Iran's state news agency IRNA said, quoting an intelligence ministry official.

It said some members of the alleged CIA network had been arrested and handed over to the judiciary, while others still required "additional investigations."

The agency's source did not specify how many agents were arrested or if they were operating only in Iran.

- 'Show restraint' -

In Moscow, President Vladimir Putin's spokesman Dmitry Peskov on Tuesday urged all sides "to show restraint" while Chinese Foreign Minister Wang Yi warned them "not to open a Pandora's box."

He urged Washington to "change its practice of extreme pressure" but also called on Tehran not to abandon the nuclear agreement "so easily."

The United States has blamed Iran for last week's attacks on two tankers in the Gulf of Oman, an accusation Tehran denies as "baseless."

The Pentagon released new images on Monday that it said showed Iran was behind the attack on one of the ships.

The US claim centers on what it describes as an unexploded limpet mine on the Kokuka Courageous tanker ship that it says was removed by Iranians on a patrol boat.

The US released a grainy black and white video last week it said showed the Iranians removing the mine, but has not provided an explanation for why they allegedly did so while the US military was in the area.

French President Emmanuel Macron appeared not fully convinced, saying that only with "all the doubts lifted can the attributions (of blame) be made."

Tehran has vehemently denied any involvement and hints that Washington itself could be responsible.

- Iran ultimatum -

Iran's atomic energy organization said Monday the country would soon pass the amount of low-enriched uranium allowed under the nuclear deal.

The move "will be reversed once other parties live up to their commitments," said spokesman Behrouz Kamalvandi.

President Hassan Rouhani announced on May 8 that Iran would stop observing restrictions it had agreed to in the nuclear deal, in retaliation for the US withdrawal.

Rouhani said Iran would further scale down nuclear commitments by July 8 unless remaining parties to the deal -- Britain, China, France, Germany and Russia -- help it circumvent US sanctions and sell oil.

European leaders have urged Iran to stick to the deal, which set a limit on the number of uranium-enriching centrifuges, and restricted the country's right to enrich uranium to no higher than 3.67 percent, well below weapons-grade levels of around 90 percent.

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