Zarif offered an apology for his "shortcomings" in Monday's unexpected Instagram message, with prominent members of parliament calling for Rouhani to reject 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," he 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," state run IRNA news agency quoted him as saying.

- 'Good riddance' -

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

His ready smile and mastery of both the English language and social media have made him a formidable player on the diplomatic stage.

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

Netanyahu was a bitter opponent of the 2015 nuclear deal Zarif negotiated with Barack Obama's administration and threw his own formidable lobbying powers into an ultimately successful campaign to persuade US President 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, adding that supreme leader Ayatollah Ali Khamenei "makes all final decisions".

- '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.

Semi-official ISNA news agency said 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 on Tuesday said Assad had thanked Iran's 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.

The president did not directly react to Zarif's resignation but praised him for being in the "front line against America", thanking the foreign minister for his "steadfastness".

Rouhani's chief of staff Mahmoud Vaezi said the president's remarks amounted to his "total satisfaction with Dr Zarif's performance and a tough response to some biased and incorrect analyses".

"In the view of Dr Rouhani, the Islamic Republic of Iran has only one foreign policy and one foreign minister," he added in an Instagram post.

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 to reject Zarif's resignation.

"A great majority of MPs demand that the president never accept this resignation," he said on Twitter.

Deputy parliament speaker Ali Motahari slammed Rouhani for allowing "interference in foreign policy" and called on him to show authority in defending Zarif and the ministry.

Hardliners were not as supportive, indicating a deepening rift between Iran's political factions.

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

"It is not political chivalry to avoid accountability... Zarif must stay," parliament's ultra-conservative Velayi faction said in a statement.

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 face-off between Zarif and the hardliners has intensified 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 negotiations 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 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 pitted the government against parliament and a key arbitration body.

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

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