Finance ministers gathered at the United Nations said that, with millions of dollars coming from illegal oil sales, it was time to target Islamic State funding.

"So far, ISIL has reaped an estimated $500 million dollars from black market oil and millions more from the people it brutalizes and extorts," U.S. Treasury Secretary Jacob Lew said in a statement from the United Nations. The Islamic State jihadist organization is also referred to as Daesh and by the acronyms ISIL and ISIS.

The U.S. State Department in September offered a $5 million reward for justice for tips that would help thwart the sale of oil and antiquities to finance terrorist operations. As of July, the terrorist group, which claims control over parts of Iraq and Syria, has brought in as much as $10 million dollars through a black market for oil.

The funds have been used to pay monthly salaries to its militants and provide stipends to their families.

Risk consultancy group Verisk Maplecroft confirmed the group's oil income already declined from $1.6 million per day last year to around $500,000 a day. Military strikes on Islamic State targets have damaged regional energy infrastructure, from makeshift refineries to fuel convoys.

Over the past month, the U.S. government said nearly 400 oil tanker trucks commissioned by terrorist groups in Iraq and Syria have been destroyed in military airstrikes.

"While these attacks are having a real and growing impact, the United States and international community must also work with countries bordering Iraq and Syria to enhance border security to help stop illicit cross-border flows," the U.S. treasury secretary said.

Lew chaired a special session of finance ministers gathered to address terrorist financing. A joint statement from the meeting said the United Nations would use U.N. Security Council resolutions targeting al-Qaida as a means to address finance operations of the Islamic State.

"This resolution is a critical step, but the real test will be determined by actions we each take after adoption," he said. "We need meaningful implementation, coordination, and enforcement from each country represented here, and many others."