The US Justice Department on Thursday announced the creation of a special task force to investigate what it called "narcoterrorism" by the powerful Lebanese movement Hezbollah.

The unit will comprise specialists on money-laundering, drug trafficking, terrorism and organized crime, targeting Iran ally Hezbollah's sprawling network, whose reach extends across Africa and into Central and South America, the department said.

"The Justice Department will leave no stone unturned in order to eliminate threats to our citizens from terrorist organizations and to stem the tide of the devastating drug crisis," said Attorney General Jeff Sessions.

"The team will initiate prosecutions that will restrict the flow of money to foreign terrorist organizations as well as disrupt violent international drug trafficking operations."

The move comes amid a stepped-up effort to battle Iran's growing influence in the Middle East and the expanded military capabilities of Hezbollah, a dominant force in Lebanese politics.

But Sessions said the creation of the Hezbollah Financing and Narcoterrorism Team was also a response to allegations that former president Barack Obama held back from cracking down on Hezbollah's global networks, investigated under the previous Project Cassandra, in order to achieve the nuclear deal with Iran.

"The HFNT will begin by assessing the evidence in existing investigations, including cases stemming from Project Cassandra, a law enforcement initiative targeting Hezbollah's drug trafficking and related operations," the department said.

Officials in Washington and US allies Saudi Arabia and Israel, have increasingly raised the alarm over Hezbollah's growing power in Lebanon and around the world.

On Wednesday, former top Treasury Department sanctions official Juan Zarate told Congress that Hezbollah's drug smuggling and money laundering operations are global in scale.

"Recent actions by the Drug Enforcement Administration (DEA) and Treasury to dismantle networks of Hezbollah's 'Business Affairs Component' have exposed financial and trade nodes that the Hezbollah operates and led to arrests and enforcement actions around the world," he told a hearing of the House Foreign Affairs Committee.

- Global money-laundering -

The US has long targeted Hezbollah with sanctions. A 2007 presidential order allowed the seizure of property of "persons undermining the sovereignty of Lebanon," not naming the group but clearly aimed at it.

In 2011 the Obama administration unleashed a crackdown on the group's far-flung associates, designating Beirut-based Lebanese Canadian Bank a "primary money laundering concern" for handling the funds of alleged Hezbollah drug kingpin Ayman Joumaa.

The US Treasury and Drug Enforcement Administration later described a massive operation involving Colombia and Panama-based drug traffickers shipping multi-tonne amounts of cocaine to the United States, Europe and elsewhere.

The network laundered billions of dollars of their own money and that of other traffickers through Panama shell companies, various banks in Lebanon and elsewhere, and an operation that exported tens of thousands of used cars from the United States for sale in West Africa.

In 2012 Hezbollah chief Hassan Nasrallah denied the allegations, saying that the group depended on Iran for funding and that drug trafficking is forbidden by Islam.

But according to former DEA official Derek Maltz, the business continues. Hezbollah most recently used the proceeds to buy weapons for the group's operations in Syria, and some of the funds have also made their way to Yemen, where Iran-backed rebels are battling Saudi-supported government forces for control of the country, he said.

Speaking at the Foundation for the Defense of Democracies think tank Thursday, Maltz cheered Sessions' decision.

"Action speaks much louder than words. You saw first-hand the Attorney General of the United States took immediate action to address this issue. So I want to thank him publicly, to recognize this is leadership we need in this country."