(CNN) Iran's foreign minister, Mohammad Javad Zarif, who played a key role in the 2015 nuclear deal, announced his resignation Monday.
"I sincerely apologize for the incapacity to continue serving and all the shortcomings during the service," Zarif wrote on his Instagram account, adding, "Be happy and upbeat."
On Tuesday morning, he urged foreign ministry employees and Iranian diplomats not to follow suit. "I humbly urge all my colleagues in the Foreign Ministry and Iran's foreign missions to be fully committed to defending the country and avoid taking such measures," Zarif said in a message to the ministry's personnel, Iran state media IRNA reported.
Iranian President Hassan Rouhani has not yet accepted Zarif's resignation, the top diplomat added.
Deputy foreign ministry spokesman Sayyed Abbas Moussawi confirmed Zarif's resignation in an interview with IRNA.
Zarif did not give a reason for his resignation in his announcement.
Earlier on Monday, Syrian President Bashar al-Assad visited Iran on his first publicized official visit to the country since 2010 and the Iranian top diplomat did not appear in official photographs of the meetings. Assad met with Iran's Supreme Leader Ali Khamenei, and separately, with Rouhani.
In both meetings, the leader of Iran's elite Revolutionary Guards Quds Force, Qasem Soleimani, appeared alongside Assad. Soleimani is widely credited with being the mastermind of Iran's campaign to prop up Syria's embattled president during the country's ongoing war.
Zarif was the key architect in the landmark deal aimed at reining in Iran's nuclear program -- which US President Donald Trump withdrew from last year. He has been Iran's foreign minister since 2013.
Zarif is well known in western circles. He was educated in the United States and speaks English fluently.
Zarif left Iran in 1977, received his undergraduate degree from San Francisco State University in 1981, his master's in international relations from the University of Denver in 1984 and his doctorate from the University of Denver in 1988. Both of his children were born in the United States.
Zarif's resignation could mark the end of efforts within Iran to craft a foreign policy centered on engagement with the West, some analysts said.
They pointed to the Trump administration's hostility to the Iran nuclear deal, known formally as the Joint Comprehensive Plan of Action, as one possible reason for the Zarif's resignation.
The JCPOA, which Europe, China, Russia and Tehran still adhere to, promised Iran economic benefits and relief in exchange for restraints on its nuclear program.
The Trump administration's withdrawal from the deal and its subsequent aggressive use of sanctions against Iran has severely undercut the economic benefits Iran expected, leading to frustration within the country.
"If Zarif ends up being replaced, at least one dimension of this is that the Trump administration's war on the JCPOA inevitably led to political casualties in Iran" among the deal's supporters, said Trita Parsi, former president of the National Iranian American Council.
He noted that another champion of the nuclear deal, European Union foreign policy chief Federika Mogherini, is also set to step down.
"This would, of course, come only weeks before Mogherini's tenure in Europe ends," Parsi said. "That could spell an even darker prospect for the JCPOA -- and an Iranian foreign policy centered on diplomacy and engagement with the West."
Parsi cautioned that Zarif "has handed in his resignation, but that doesn't mean that this is the end of this story."
"It will be very interesting to see the reactions of the Iranian public to this decision," Parsi said. He noted that Zarif had given remarks at the Munich Security Conference, an international gathering, and his performance "was very well received in Iran, even among the hardliners. So his domestic position seems to be relatively strong."
Some analysts said that Zarif's reputation as a moderate within the Iranian system made little difference, given the nature of the regime.
"Javad Zarif represented hope above reality," said Mark Dubowitz, chief executive of the Foundation for Defense of Democracies, which lobbied hard against the Iran nuclear deal that Zarif negotiated with then-Secretary of State John Kerry and European partners.
"Looking for moderation where there was none, too many in the West forgot that, behind his smiling face and deceitful claims, the regime he represents is brutal, corrupt and destructive," Dubowitz said.