Officials said the recently intercepted message was sent by Qasem Soleimani, head of Revolutionary Guards' Qods Force, to Iranian-supported Shiite militias in Iraq, The Wall Street Journal reported Thursday.
In it, Soleimani said Shiite groups must be ready to respond with force should a U.S. airstrike on Syria occur.
The Journal said Iranian officials didn't respond to its request for comment.
Iraqi Shiites have shown favorability toward the Alawite-dominated government of Syria and oppose U.S. strikes against President Bashar Assad's regime.
U.S. officials said the American Embassy in Baghdad was a likely target but didn't describe other potential targets in Iraq, the Journal said.
Military officials have been trying to determine the range of possible responses from Syria, Iran and their allies if the United States launches a military strike in response to evidence that the Assad regime used chemical weapons against Syrians in August in an attack on Damascus suburbs.
The State Department issued an alert Thursday warning U.S. citizens against non-essential travel to Iraq and citing terrorist activity "at levels unseen since 2008."
In Tehran, Iran's new president says his country will send humanitarian aid to Syria if Washington attacks it, but didn't repeat an earlier hardliner vow to attack Israel.
At the same time, Supreme Ruler of Iran Ali Khamenei said the United States would make a mistake in attacking Syria and would "definitely suffer" as a result.
"If something happens to the Syrian people, the Islamic Republic of Iran will do its religious and humanitarian duties to send them food and medicine," President Hassan Rouhani told Iran's Assembly of Experts, a deliberative body of Islamic theologians that elects and removes Iran's supreme leader and supervises his activities.
Khamenei, as supreme ruler, is Iran's head of state and highest ranking political and religious authority.
Rouhani, an Assembly of Experts member since 1999, described the situation in Syria as "dire" and condemned "military attacks on countries in this region, especially on Syria," but made no mention in his remarks of retaliation against Israel.
The commander in chief of Iran's elite Revolutionary Guard Corps said last week an attack on Syria would lead to the "destruction of Israel."
Khamenei said in remarks reported by state-run Press TV U.S. officials had no right to make "humanitarian claims [given] their track record" in Iraq, Afghanistan and at the military prison in Guantanamo Bay, Cuba.
Diplomats and analysts said they were not sure if Rouhani's moderate message represented a significant policy shift in Iran.
Nader Karimi Joni, an independent analyst and chief editor of the Iranian sociopolitical magazine Gozaresh, told the Los Angeles Times he interpreted Rouhani's reserved offer as following through on campaign promises to give priority to Iranians' desire for improved living conditions and an end to international isolation.
The Times said Rouhani appeared to be outmaneuvering hard-liners in curbing overblown vows to defend Syria from threatened U.S. airstrikes.
Iran, along with Russia, remains Syria's most important ally, a source of material support and intelligence.
Iran's state-run media increasingly broadcast programs reminding people Iranians suffered chemical weapons attacks during the 1980-1988 war with Iraq, The Washington Post reported.
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