Senators: ISIS is ‘best funded’ terror group ever

Sens. Marco RubioMarco RubioObama plans 'aggressive' blitz for Clinton in campaign's final days One way or another, 2016 was all about Donald Trump's hands Poll: Clinton has 4-point lead over Trump in Florida MORE (R-Fla.) and Bob CaseyBob CaseySenate Dems want major women's golf event moved off Trump course 5 takeaways from the Pa. Senate debate Great Lakes senators seek boost for maritime system MORE Jr. (D-Pa.) are urging the Obama administration to try and cut off the Islamic State in Iraq and Syria’s financing, calling it the “best funded terrorist group in history.”

"ISIS’s criminal activities — robbery, extortion, and trafficking — have helped the organization become the best funded terrorist group in history," wrote Rubio, who sits on the Foreign Relations and Intelligence committees, and Casey, a member of the National Security Working Group, in a letter Tuesday to Secretary of State John KerryJohn KerryThe Atlantic Council's questionable relationship with Gabon’s leader State Dept. months late on explaining Clinton aide's missing emails The evidence backs Trump: We have a duty to doubt election results MORE.

"This wealth has helped expand their operational capacity and incentivized both local and foreign fighters to join them," they added. "ISIS has the resources, weaponry, and operational safe havens to continue to threaten the stability of the region and U.S. national security interests." 

In the letter, the senators said ISIS was using black markets, smuggling routes and third-party groups to sell oil to finance their operations. They asked the State Department to designate ISIS as a "Transnational Criminal Organization." 

That designation would "send a strong signal to other countries and potential third parties" who are buying oil from ISIS on the black market, they wrote. 

ISIS could be earning up to $2 million a day through the illicit sale of oil from reserves captured in Iraq and Syria, they said. 

The senators said "some government officials in the region" have helped facilitate the trade. Although much of that oil ends up back in Syria, including with strongman Bashar Assad’s regime, some of it is "making its way to third countries." 

Rubio and Casey said they were also concerned that ISIS is receiving "significant financial support from private individuals residing in several U.S. partner countries in the region.” 

They called for the State Department to outline measures being taken to ensure countries in the region are targeting smuggling networks that do business with ISIS in violation of international sanctions. 

"We urge you to make the halting of these private financial flows a greater priority and to make clear to countries in the region that continued financial support of ISIS could lead to imposition of financial penalties on their citizens and financial institutions by the U.S. Government," they wrote.