According to senior US treasury official Adam Szubin the group has accumulated up to $1.5bn from looting banks in Iraq and Syria, and from black market oil sales, reported the Al-Jazeera.
"ISIS has made more than $500m from black market oil sales. It has looted between $500m and $1bn from bank vaults captured in Iraq and Syria," Szubin said.
They also extorted millions of money from people living in the areas they control.
"US has placed financial sanctions on more than 30 "ISIS-linked senior leaders and financiers in 2015 only," said Szubin.
"ISIL is wealthy. But it has its vulnerabilities. Waging a multi-front war while attempting to essentially act as a proxy-state requires steady and renewable sources of funding," he said.
"We are targeting both of these dependencies ISIS's ability to generate revenue, and its ability to use that revenue."
His statement has come at the time when Pentagon confirmed death of group's financial minister, Abu Salah. He was killed in November.