ISTANBUL — Iranian Foreign Minister Mohammad Javad Zarif, known for negotiating a nuclear deal with world powers, abruptly tendered his resignation via Instagram late Monday, Iran’s state news agency reported.

The move by Zarif, a well-liked diplomat both at home and abroad, could upend Iran’s foreign policy at a critical time for the Islamic republic, which is suffering from renewed U.S. sanctions.

“I highly appreciate the graciousness of the brave Iranian people and respected officials over the past 67 months,” Zarif wrote in the statement, which was posted to his verified Instagram account.

“I sincerely apologize for my lack of ability to continue my service and for all of the shortcomings,” he added.

Syrian President Bashar al-Assad met with Iran’s Supreme Leader Ayatollah Ali Khamenei Feb. 25. Editor’s note: this video has no audio.

The reason for Zarif’s resignation was not disclosed. Nor is it clear whether top Iranian officials will accept the resignation, and there were immediately conflicting reports about whether President Hassan Rouhani had already rejected it.

“He has handed in his resignation, but that doesn’t mean that this is the end of this story,” said Trita Parsi, president of the National Iranian American Council in Washington.

“It will be very interesting to see the reactions of the Iranian public to this decision,” he said.

Both Rouhani and Zarif have come under fire from domestic opponents following the U.S. withdrawal from the 2015 nuclear pact. The Trump administration last year abandoned the accord with Iran, which agreed to curb its atomic energy program in exchange for relief from U.S. trade restrictions.

Rouhani, a relative moderate and pragmatist, had championed the accord as his administration’s signature achievement. Zarif, who was educated in the United States and speaks fluent English, was the face of the campaign and became known for his “smiling diplomacy.”

Michael Singh, managing director of the Washington Institute for Near East Policy, said Zarif’s resignation is not surprising because the policies he championed have fallen out of favor since the U.S. withdrawal from the Iran nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).

“The Supreme Leader [Ayatollah Ali Khamenei] has been very critical of the JCPOA and outreach to the West after the U.S. withdrawal,” Singh said. “And Zarif is the poster boy for the JCPOA and engagement with the West.”

Singh said Zarif’s departure may be rooted in domestic issues, rather than foreign policy, because there are no significant prospects for negotiations on either Iran’s nuclear program or on the issues of concern to Europeans and Americans. Singh noted that Rouhani has been involved in a power struggle with hard-line elements in the government and that Zarif may be the casualty.

“Absent Rouhani stepping down — which would be very significant, to end his presidency early — Zarif is bearing the brunt of this and is ultimately the one who’s lost his job over this.”

Morello reported from Washington. Kaveh Nematipour contributed reporting from Istanbul.