"If any disruption happens regarding the sale of Iranian oil, the Strait of Hormuz will definitely be closed," Mohammad Kossari, deputy head of the Iranian Parliament's foreign affairs and national security committee, declared after the European Union agreed Monday to impose a phased ban on oil purchases from Iran and Washington expanded its sanctions to include Iran's third-largest bank.
Kossari was one of two Iranian lawmakers to threaten to block the only sea passage to the open ocean for large areas of the petroleum-exporting Persian Gulf.
EU countries agreed not to sign new oil contracts with Iran and to end existing ones by July 1, foreign ministers meeting in Brussels said in a statement. The European market accounts for about a fifth of Iran's oil exports.
The EU countries agreed to freeze the Iranian central bank's assets in Europe and to ban Iranian gold, precious metals and diamond transactions, effective Tuesday.
Washington's move, intended to bolster the EU sanctions, targets Bank Tejarat for its alleged role in financing Iran's nuclear program and for assisting other Iranian banks and companies in evading international sanctions.
It is the 23rd financial institution in Iran subject to U.S. sanctions.
The Tejarat sanctions strike "at one of Iran's few remaining access points to the international financial system," Treasury Undersecretary for Terrorism David Cohen said in a statement.
The sanctions "will deepen Iran's financial isolation, make its access to hard currency even more tenuous and further impair Iran's ability to finance its illicit nuclear program," he said.
The U.S. sanctions follow President Barack Obama's move last month to ban any U.S. dealings with Iran's central bank.
Iran's currency, the rial, has lost more than 70 percent of its value against the U.S. dollar since September.
After the Iranian lawmakers threatened to close the Strait of Hormuz, which includes Iranian territorial waters, U.S. Ambassador to NATO Ivo Daalder vowed Washington and its allies would take whatever action was needed to keep the narrow waterway open to international shipping and the oil business.
The strait "needs to remain open and we need to maintain this as an international passageway," he told the BBC. "We will do what needs to be done to ensure that is the case."
About 13 tankers carrying 15.5 million barrels of crude oil -- about a third of the world's tanker-borne oil and about 17 percent of all world oil shipments -- pass through the strait on an average day. The tankers' cargo include crude from Iran, Iraq, Kuwait, Saudi Arabia and the United Arab Emirates.
Daalder said Washington and other Western countries were "ready at any time to sit down and have a serious conversation with [Iran] to resolve this [nuclear] issue with negotiations."
The ball is in Iran's court, he said, with Tehran "either finding a way to cooperate and negotiate a resolution to this issue, which we are willing to do, or choosing a path of isolation."
"I have not looked at the exact military contingency plannings that there are. ... But of this I am certain -- the international waterways that go through the Strait of Hormuz are to be sailed by international navies, including ours, the British and the French and any other navy that needs to go through the Gulf," he said. "And second, we will make sure that that happens under every circumstance."
Western politicians allege Iran is building a nuclear weapons capability, but Tehran insists its nuclear program is for civilian uses only.