javad Zarif, a pivotal player in the nuclear deal, has been educated and lived for years in US (Reuters)

Iran accused the United States on Thursday of "childish behaviour" driven by fear after Washington imposed sanctions on its foreign minister, fanning tensions between two foes at loggerheads over Gulf shipping and Iran's nuclear programme.

Fears of a Middle East war with global repercussions have risen since the United States ditched world powers' 2015 nuclear deal with Iran last year and revived sanctions on Tehran.

The Islamic Republic has retaliated by resuming uranium enrichment seen in the West seen as a potential conduit to developing atomic bombs. Iran denies having any such objective.

After several attacks in May and June on oil tankers - blamed by Washington on Tehran, which denied responsibility - U.S. President Donald Trump has been trying to forge a military coalition to secure Gulf waters, though European allies have been loath to join for fear of provoking open conflict.

European parties to the nuclear pact have called for diplomacy to defuse the crisis, but Tehran and Washington have taken hard lines and on Wednesday the Trump administration slapped sanctions on Iran's foreign minister - a likely further blow to any chances for troubleshooting dialogue.

"They (Americans) are resorting to childish behaviour... They were claiming every day 'We want to talk, with no preconditions' ...and then they sanction the foreign minister," Iranian President Hassan Rouhani said on state television.

Zarif, a pivotal player in the nuclear deal who was educated and lived for years in the United States, dismissed the action and said it would not affect him as he had no property or other interests in America.

U.S. Afraid Of Zarif's Words, Iran Says

"A country which believes it's powerful and a world superpower is afraid of our foreign minister's interviews," Rouhani said, alluding to numerous interviews that Zarif - a fluent English speaker - gave to American media when he visited New York for a United Nations conference in July.

"When Dr Zarif gives an interview in New York...they (Americans) say Iran's foreign minister is misleading our public opinion," Rouhani said. "What happened to your claims of liberty, freedom of expression and democracy?"

Rouhani added: "The pillars of the White House are made to tremble by the words and the logic of a knowledgeable and self-sacrificing man and diplomat."

U.S. Treasury Secretary Steven Mnuchin said Zarif was being sanctioned because he "implements the reckless agenda of Iran's Supreme Leader...(We are) sending a clear message to the Iranian regime that its recent behaviour is completely unacceptable".

In a tweet earlier on Thursday, Zarif said peace and dialogue were an "existential threat" to hawkish politicians allied with U.S. President Donald Trump who take a hardline stance against the Islamic Republic.

Zarif has in the past said that a so-called "B-team" including Trump's national security adviser John Bolton, an ardent Iran hawk, and conservative Israeli Prime Minister Benjamin Netanyahu could goad Trump into a war with Tehran.

In repudiating the nuclear deal reached by predecessor Barack Obama, Trump said he wanted to secure a wider accord that not only limited Iran's nuclear activity but also curbed its ballistic missile programme and reined in its support for powerful proxies in Syria, Iraq, Yemen and Lebanon.

Trump intensified sanctions in May to try to strangle Iran's oil exports, the lynchpin of its economy.

Fears For Gulf Oil Shipping

The security of shipping in the Gulf, through which about a fifth of the world's oil passes, has shot up the international agenda since May when Washington accused Iran of being behind explosions that holed six tankers over several weeks.

In July, Iran seized a British-flagged tanker in the Strait of Hormuz, the Gulf's outlet to the open seas, in apparent retaliation for Britain's seizure of an Iranian ship accused of violating European sanctions by taking oil to Syria.

Britain on Thursday ruled out a swap of the two tankers. "We are not going to barter: if people or nations have detained UK-flagged illegally then the rule of law and rule of international law must be upheld," Foreign Secretary Dominic Raab said.

In a sign of increasing jitters over security in the Gulf, Royal Dutch Shell said on Thursday it was not taking any British-flagged tankers through the Strait of Hormuz for the time being.

The U.S. Embassy in Berlin said on Tuesday the United States had asked Germany to join France and Britain in a mission to protect shipping transiting the strait and "combat Iranian aggression". Germany rebuffed the request.

"Germany will not take part in the sea mission presented and planned by the United States," Foreign Minister Heiko Maas said. The Gulf situation was serious and everything should be done to avoid conflict, he said. "There is no military solution."

Russia, another party to the 2015 deal, said on Thursday it had the impression the United States was seeking a pretext for conflict in the Persian Gulf, RIA news agency reported.

"Events there are really moving to a dangerous point and there are risks of a large-scale military clash," RIA cited Foreign Ministry spokeswoman Maria Zakharova as saying.