The United States has announced new sanctions on two Iran-backed militias fighting in Syria in a move aimed at raising pressure on Tehran and the powerful Islamic Revolutionary Guard Corps (IRGC), as Washington prepares for a military withdrawal from the war-torn country.

The Fatemiyoun Division, comprising Afghan nationals, and the Zaynabiyoun Brigade, comprising Pakistani fighters, were placed on the US Treasury's financial blacklist, which aims to cut off their access to international financial networks to choke their operations.

Both militias are recruited by Iran's elite military unit, the IRGC, the Treasury said, from communities of refugees and migrants living inside Iran, and sent to fight for the Bashar al-Assad regime in Syria.

"The brutal Iranian regime exploits refugee communities in Iran... and uses them as human shields for the Syrian conflict," Treasury Secretary Steven Mnuchin in a statement.

"Treasury's targeting of Iran-backed militias and other foreign proxies is part of our ongoing pressure campaign to shut down the illicit networks the regime uses to export terrorism and unrest across the globe."

In a post on social media, US Secretary of State Mike Pompeo accused Iran of "preying" on refugees and using them as "cannon fodder in Syria".

"Maximum pressure continues," he wrote on Twitter.

Since President Donald Trump announced that Washington will withdraw its roughly 2,000 troops in Syria, the administration has tried to allay concerns that fighters from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group could stage a comeback in the region, or that Iran and Russia will benefit from the US departure.

Today, the U.S. Treasury took action against two Syria-based, #Iran-backed militias composed of foreign nationals: The Fatemiyoun and Zeinabiyoun.



Who are these militias? @ColinPClarke & I explained in our latest essay in @ForeignAffairs. https://t.co/RGruWYyPyD — Ariane Tabatabai (@ArianeTabatabai) January 24, 2019

The Treasury also designated for sanctions Qeshm Fars Air, an airline it said was controlled by already-sanctioned Mahan Air, and Armenia-based Flight Travel LLC, which markets for Mahan.

Qeshm Fars was a dormant carrier until 2017 when it was revived to deliver passengers and cargo to Damascus from Iran with two Boeing 747 aircraft on behalf of the IRGC.

The carrier is being staffed by Mahan Air employees and receives Mahan technical support, the Treasury said.

"We are aggressively targeting those who continue to provide commercial support to Mahan Air and other designated airlines, and any who fail to heed our warnings expose themselves to severe sanctions risk," Treasury Undersecretary for Terrorism and Financial Intelligence Sigal Mandelker was quoted in the statement as saying.

As a result of the latest sanctions, all property and interests in property of these entities that are in the US or in the possession or control of US persons are blocked.

In addition, persons and foreign financial institutions that "knowingly" engage in certain transactions with the entities designated today may themselves be exposed to sanctions.