How Iran is skirting sanctions, and where U.S. officials are following the money

World powers resume negotiations with Iran next week over its nuclear program. Secretary of State John Kerry is asking Congress to give diplomacy a chance. He met Wednesday with lawmakers on Capitol Hill.

Kerry is asking them to hold off on efforts to impose tougher economic sanctions on Iran: "Our hope is that no new sanctions would be put in place for the simple reason that, if they are, it could be viewed as bad faith by the people we are negotiating with... It could destroy the ability to be able to get agreement."

But some Republican leaders are siding with Israel, which sees Iran as an existential threat to its very existence, and has pushed hard for new sanctions.

Sen. Mark Kirk, of Ill., called the administration's closed-door presentation "very unconvincing" and "fairly anti-Israel."

The level of sanctions and financial pressure on Iran is unprecedented, but despite those attempts to isolate the country, some believe Iran's nuclear program is continuing to expand.

Iran is already the most sanctioned country in the world. The punishment has devastated its economy, but Iran's leaders have found ways to evade sanctions and have amassed billions in off-the-book investments that the U.S. government is now hunting down, CBS News' Margaret Brennan reported on "CBS This Morning."

The supreme leader -- Ayatollah Ali Khamenei -- has the ultimate say on everything in Iran, including the government, the economy, the military and the country's nuclear program. He's also been immune to the sanctions that have crippled the country, thanks to a hidden economy that's flourished under his rule.

According to the U.S. Treasury, Khamenei funneled billions of dollars through a holding company known as Setad; a network of 37 businesses that include construction companies, oil companies, banks and insurance firms. The profits are hidden throughout Europe, Africa and the Middle East and controlled by the Ayatollah. The U.S. argues that some of the proceeds fund terrorism and the country's nuclear advances.

Juan Zarate, a former White House official and author of "Treasury's War: Unleashing a New Era of Financial Warfare," said, "With a regime that controls the economy, that controls investment vehicles and key elements of the industry and economic infrastructure of the state, it allows the clerical establishment and the Revolutionary Guard (elite military unit), to profit and make money and to try to move and hide it as well."

The U.S. Treasury exposed Setad in June, and tried to shut it down. One of its holdings, Rey Investment, is worth $40 billion -- much of it thanks to embezzled donations meant for religious shrines. Another company known as Tadbir hid hundreds of millions in insurance companies and banks, according to the Treasury. That's where the Ayatollah is said to hide his own money.

"This is a game of cat-and-mouse, a modern form of financial warfare that the Treasury and others in the West have been trying to engage in, with the Ayatollahs and the clerical establishment trying to raise and move money, not only for their personal benefit, but for the survival of the regime and the development of a nuclear program," said Zarate.

"Skeptics, including the Israeli government, argue that the sanctions are not strong enough to force Iran's leaders to halt the nuclear program. The Obama administration defends itself, saying existing sanctions have slowed down nuclear development, and they have succeeded in bringing Iran to the negotiating table," said Brennan.