The United States imposed sanctions on Wednesday on a gang it alleges has smuggled migrants out of war-torn Syria and through Mexico to the southern US border.

The US Treasury accused a 47-year-old Syrian, Nasif Barakat, of operating a “global network” bringing travellers to the United States illegally.

Would-be migrants pay an average of $20,000 and are provided with false or fraudulently-obtained documents, sometimes including European passports.

Their route begins in Syria and transits Lebanon, Turkey and the United Arab Emirates, before arriving in “certain South and Central American countries” and then the southwest US border.

Under the sanctions, US citizens are forbidden from doing business with Barakat or his gang, which is designated a “transnational criminal organisation.”

Attorney General Jeff Sessions declared in the US statement that the action follows an executive order from President Donald Trump to dismantle such gangs.

“Many of these organisations use human smugglers to bring people across our borders with little regard to their safety or our national sovereignty,” Sessions said.

“We will continue to use every lawful tool we have to defend our borders, fight alien smuggling, restore legality to our immigration system, and to defund criminals of their ill-gotten gains.”

Syria has been in the grip of a multi-faction civil war for the past seven years, and the fighting has drawn in troops from Iran, Russia and a US-led coalition.

At the weekend, Trump ordered a brief Western air bombardment to punish Bashar al-Assad's regime for its alleged use of banned chemical warfare agents against civilians.

But under Trump, the number of Syrian refugees that have been allowed into the United States legally has plummeted to 44 this financial year from 6,557 in 2017.