Intelligence officials and diplomatic sources say that Iran has smuggled at least $1 billion in cash into the country to blunt the effects of Western sanctions.

Multiple sources told Reuters on Tuesday that Iran’s efforts to obtain smuggled bank notes for international transactions is much bigger than previously thought.

How has it been done so far? Western and Iranian sources said Iran’s central bank had in recent months worked with other entities, including other sanctioned Iranian companies, to find ways to obtain U.S. dollars, including using front companies and their networks. They said the central bank had given the orders to the front companies abroad to buy dollars. The sources said the cash was hand-carried by couriers on flights from Dubai or Turkey, or brought across the Iraq-Iran border. Before it reached Iran, the cash was passed through money changers and front companies in Dubai, in the United Arab Emirates, and Iraq, they added.

News of the smuggling efforts comes as the U.S. and its allies scramble to reach a nuclear deal with Iran by a March 31 deadline. The next round of discussions are scheduled for March 2.

The U.S. Treasury reported on Iran’s efforts to skirt sanctions in December, but at the time the figure was thought to be in the hundreds of millions of dollars.

Reuters interviewed three Western diplomatic sources and three Iranian government officials, all who said that Iran has been working on better ways to evade sanctions over its nuclear program since last March.

Officials said that Iran is using front companies in Dubai, Iran and in the United Arab Emirates to obtain dollars, Reuters reported. The news service’s sources asked not to be identified due to the sensitive nature of details disclosed.

Sources: Reuters and Washington Times