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“There has been a steady increase in our sanctions activity, and this is part of that escalation,” he said.

Mr. Carney said U.S. sanctions on Iran already are squeezing Iran‘s economy and have exacerbated tensions within the Iranian leadership.

“There is no question that the impact of the isolation on Iran and the economic sanctions on Iran have caused added turmoil within Iran,” he said.

Iran is the world’s third-largest exporter of crude oil, giving its leaders financial resources and leverage to withstand outside pressure. Last year, Iran generated $100 billion in revenue from oil, up from $20 billion a decade ago, according to IHS CERA, an energy consulting firm.

If Iranian oil is prevented from getting to market, other suppliers could make up the difference. The U.S. has been pressuring other Middle East and African nations to step up production for sale to Europe. Saudi Arabia has said it could increase production to make up for any lost Iranian crude.

Iran‘s disputed nuclear program became a global concern more than five years ago when the extent of the country’s research and uranium enrichment began to be known. Since then a web of international economic and other sanctions have failed to stop Iran‘s progress toward a point that it could build one or more nuclear devices.

U.S. intelligence agencies say Iran is indeed close to that ability but has not yet decided to go ahead. Iran says its nuclear program is peaceful and denounces sanctions as aggression.

The White House previously has said it would take months to evaluate the likely effect on the fragile global economy before taking the next large steps, including new penalties on the central bank.

Now, U.S. institutions are required to seize any Iranian state assets they come across, rather than rejecting the transaction involved.

The value of Iranian assets affected by the new order was not clear. Iran does almost no direct business with the United States after three decades of enmity, but its money moves through the world financial system and its oil is sold in dollars.