(CNN) The war against ISIS is expanding to a new frontier: its finances.

As the terror group plunders archaeological sites and oil fields in Iraq and Syria, the United States is offering a $5 million reward for tips to stop the smuggling of gas and antiquities.

The terror group's oil and antiquities trafficking are crucial sources of revenue, the State Department said in a statement announcing the reward.

Funds for brutal tactics

ISIS makes millions of dollars from the trafficking of oil and antiquities, which helps fund its brutal tactics, U.S. officials said.

"The U.S. Department of State hopes this reward generates information regarding individuals or entities engaged in the production, facilitation, processing, smuggling, distribution, sale and trade of oil and antiquities that benefit ISIL, as well as information regarding smuggling networks, methods and routes," it said.

The terror group uses various acronyms, including ISIL, which stands for the Islamic State of Iraq and the Levant.

Experts have described it as the best-funded terrorist organization in history , which draws recruits from all over the world.

Salaries for fighters

The funds are used for various things, including paying the terror group's fighters.

The reward announcement came the same day a bipartisan congressional task force concluded the United States is losing the battle to stop Americans from traveling abroad to enlist in ISIS.

More than 25,000 foreigners have flocked to Syria and Iraq since 2011 to fight with ISIS and other Islamist terrorist groups, according to U.S. estimates.

And despite declining finances, ISIS still boasts of hefty incomes.

Its oil income alone has plunged to $500,000 a day from as much as $1.6 million a day last year, according to risk consultancy group Verisk Maplecroft.