State Department Farsi spokesman Alan Eyre expressed his condolences over the death of Efat Kashani on his Facebook page, which has over 70,000 likes. He also shared the Internet link to his message via Twitter.
Eyre wrote that he heard about Kashani's death with the "deepest regret" and added that he prayed God might give the family "patience."
The United States has extensively used the public-diplomacy tools at its disposal to reach out to Iranians.
But Eyre's message is a rare -- perhaps unprecedented -- instance of the State Department using social media to reach out to an Iranian government official.
Zarif's mother passed away on December 27 in a Tehran hospital, according to reports in Iranian media.
In his condolences message, Eyre cited in Arabic a well-known phrase from the Koran that is often invoked when someone dies.
"We belong to God and to Him shall we return," Eyre wrote.
The message comes as Iran and the United States and its allies seek a lasting solution to the crisis over Tehran's controversial nuclear activities.
In recent months, Iranian officials have used social media sites that are blocked inside Iran to reach out to the world and prove that the tone of foreign policy in Tehran has changed since the election of relative moderate, cleric Hassan Rohani to the presidency.
Eyre's Facebook message has prompted hundreds of reactions from Iranians on social media.
"As an Iranian, I thank you for your your very diplomatic behavior and human behavior," wrote one user.
Another said: "Thank you and I would thank you even more if you could you use less Arabic words."
Among the many reactions, there is also criticism of Eyre for reaching out to an Iranian government official.
"Have you also sen[t] your condolences to the mother of Neda Agha Soltan?" asked one user in a reference to a protester gunned down during 2009 protests over the reelection of President Mahmud Ahmadinejad.
Eyre's message has been picked up and posted on several Iranian news sites, including that of Iran's official news agency, IRNA.
-- Golnaz Esfandiari