Zarif offered an apology for his "shortcomings" in the unexpected message on Instagram on Monday, with prominent members of parliament immediately calling for Rouhani not to accept the resignation.

Zarif, 59, has served as Rouhani's foreign minister since August 2013 and has been under constant pressure from hardliners who opposed his policy of detente with the West.

"I apologise for my inability to continue serving and for all the shortcomings during my term in office," Zarif said in the message posted on his verified Instagram account.

On Tuesday, he urged Iranian diplomats not to follow his lead as rumours spread of mass resignations.

"I hope my resignation will act as a spur for the foreign ministry to regain its proper statutory role in the conduct of foreign affairs," the official IRNA news agency quoted him as saying.

- 'Good riddance' -

The prospect of Zarif's departure was swiftly welcomed by Iran's foes.

The 59-year-old's ready smile and mastery of both the English language and social media has made him a formidable player on the diplomatic stage.

"Zarif is gone. Good riddance," said Israeli Prime Minister Benjamin Netanyahu.

Netahyahu was a bitter opponent of the 2015 nuclear deal Zarif negotiated with Barack Obama's administration and threw his own formibable lobbying powers into an ultimately successful campaign to persuade Donald Trump to abandon it last May.

US Secretary of State Mike Pompeo tweeted that it made no difference to Washington whether Zarif stays or goes.

"Either way, he and @HassanRouhani are just front men for a corrupt religious mafia," he said.

"We know @khamenei_ir makes all final decisions," he added, referring to Iran's supreme leader Ayatollah Ali Khamenei.

- 'No trust' -

Zarif's announcement came hours after a surprise visit to Tehran by Syrian President Bashar al-Assad, who has been a major recipient of Iranian aid during his country's nearly eight-year civil war.

According to the semi-official ISNA news agency, Zarif was not present at any of Assad's meetings with Khamenei and Rouhani.

The Entekhab news agency said it tried to reach Zarif and received the following message: "After the photos of today's meetings, Javad Zarif no longer has any credibility in the world as the foreign minister!"

Rouhani insisted on Tuesday that Assad had thanked the Iranian foreign ministry during his visit, one of his very few abroad since the start of the civil war in 2011.

"He said he has come to thank the nation and the leader of Iran. He also thanked the foreign ministry," Rouhani said.

In an interview with the conservative Jomhoori Eslami newspaper published on Tuesday, Zarif said "everything will be lost, when there is no trust in the manager of foreign policy."

Mostafa Kavakebian, a reformist MP, was among those urging Rouhani not to accept Zarif's resignation.

"A great majority of MPs demand that the president never accept this resignation," he said in a tweet.

Hardliners were not as supportive.

"Mr. Zarif resigned to avoid the hardships of confronting America and having to answer for the faulty path he took during his tenure," said Alireza Zakani, a former MP.

The head of parliament's national security and foreign policy committee, Heshmatollah Falahatpisheh, told ISNA that it was not the first time Zarif had tendered his resignation.

"That he has done so publicly this time means that he wants the president to accept it."

- 'Deadly poison' -

The faceoff between Zarif and the hardliners has intensifed as time has passed, and an attempt to impeach him in parliament was dropped only in December.

Zarif has publicly acknowledged that his main concern during the nuclear deal negoaitions had been about opposition from inside Iran.

"We were more worried by the daggers that were struck from behind than the negotiations," he told Jomhoori Eslami.

"The other side never managed to wear me down during the negotiations... but internal pressure wore me down both during and after the talks."

The latest point of contention between Zarif and the hardliners has been the implementation of the requirements of the Financial Action Task Force regarding money laundering in Iran.

The rift on the issue, which has complicated Zarif's efforts to maintain European trade and investment despite the renewed US sanctions on Iran, has pitched the government against parliament and a key arbitration body.

Zarif told Jomhoori Eslami that such partisan disputes over foreign policy were "a deadly poison."

burs-amh/kir