The six-count indictment charges Halkbank with moving billions of dollars in Iranian oil and gas proceeds back to the sanctioned country, disguising some of the transactions as involving food, medicine and other humanitarian aid.

US prosecutors on Tuesday filed criminal charges against Turkey’s state-run Halkbank for allegedly taking part in a multibillion-dollar scheme to evade sanctions on Iran.

The US prosecutors claimed that Halkbank used a web of front companies to allow Iran to spend revenues from its gas and oil sales on international markets between 2012 and 2016.

“This is one of the most serious Iran sanctions violations we have seen,” Assistant Attorney General for US National Security John Demers alleged in a statement.

Demers claimed that Halkbank had violated sanctions by “illegally giving Iran access to billions of dollars’ worth of funds, all while deceiving US regulators about the scheme.”

Tuesday’s announcement means that prosecutors believe they have gathered enough evidence to back that notion.

“High-ranking government officials in Iran and Turkey participated in and protected this scheme,” which was set up to move some 20 billion dollars of Iranian oil revenue illegally, prosecutors of the Southern District of New York said in a 45-page indictment.

Prosecutors allege that Turkish government officials received millions of dollars in bribes in return.