A threat by Iran to unleash the "full force" of its navy if a US aircraft carrier is redeployed to the Gulf, and an immediate American dismissal of the warning, sent tensions and oil prices soaring on Tuesday.
The unprecedentedly sharp rhetoric raised the possibility of conflict breaking out between arch-foes Tehran and Washington, pitching a long standoff over Iran's nuclear programme into dangerous new territory.
"We don't have the intention of repeating our warning, and we warn only once," Brigadier General Ataollah Salehi, Iran's armed forces chief, said as he told Washington to keep its aircraft carrier out of the Gulf.
The US carrier would face the "full force" of Iran's navy otherwise, a navy spokesman, Commodore Mahmoud Mousavi, told Iran's Arabic television service Al-Alam.
The White House brushed off the warning, with spokesman Jay Carney saying it "reflects the fact that Iran is in a position of weakness" as it struggles under international sanctions.
The US Defence Department said it would not alter its deployment of warships to the Gulf.
"Deployment of US military assets in the Persian Gulf region will continue as it has for decades," it said in a statement.
The developments helped send oil prices soaring more than $3 per barrel, to $102.39 a barrel for West Texas Intermediate crude and $111.02 for Brent North Sea crude.
Iran warning came after it completed 10 days of naval war games by testing three anti-ship missiles. The display of force was meant to show it controlled the strategic Strait of Hormuz, through which 20 percent of the world's oil flows.
Last week, a US aircraft carrier, the USS John C Stennis, passed through the strait and eastward, through the Gulf of Oman and a zone being used by the Iranian navy for its drill.
The nuclear-powered "supercarrier" is one of the US navy's biggest warships. It and its escort destroyers have been deployed in the Gulf for the past few months.
An Iranian military aircraft filmed the US vessel, with its bow number -- CV-74 -- visible, as it brushed past Iran's war games.
"We advise and insist that this warship not return to its former base in the Persian Gulf," Salehi was quoted as saying on the armed forces' official website.
The USS John C. Stennis started its current seven-month deployment at sea in late July 2011, according to US navy websites.
The US Defence Department did not say whether the carrier was meant to travel back into the Gulf before its scheduled return to the United States.
But it said it regularly sent one or more of its 11 aircraft carriers and their accompanying ships on rotation to the Gulf to support military operations in the region.
"Our transits of the Strait of Hormuz continue to be in compliance with international law, which guarantees our vessels the right of transit passage," it said.
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The statement also underlined the US pledge to keep the Strait of Hormuz open, saying "we are committed to protecting maritime freedoms that are the basis for global prosperity; this is one of the main reasons our military forces operate in the region."
Pentagon press secretary George Little told reporters, however: "No one in this government seeks confrontation over the Strait of Hormuz. It's important to lower the temperature."
Iran has threatened to close the strait if it comes under attack or if new sanctions hit its oil exports.
At the weekend, US President Barack Obama activated such a sanction, signing into law measures targeting Iran's central bank, which processes most of the Islamic republic's oil sales.
The European Union, which is mulling an embargo on Iranian oil, is expected to announce further sanctions of its own at the end of January.
French Foreign Minister Alain Juppe said Tuesday he wanted to see "stricter sanctions" applied on Iran.
The Western sanctions, which add to four sets of UN sanctions, punish Iran for maintaining nuclear activities that the United States and its allies believe are being used to develop a weapons capability.
The International Atomic Energy Agency published a report in November strongly suggesting Iran was researching nuclear weapons and delivery systems.
Tehran denies the allegations, saying its nuclear programme is exclusively for medical and power generation purposes.
Underlining the progress it is making, Iran's atomic energy organisation said on Sunday its scientists had made the country's first nuclear fuel rod from domestically mined uranium.
The Islamic republic has been responding to the pressure by both threatening a military response and offering to resume negotiations over its nuclear programme that were suspended nearly a year ago.
Iran was waiting for EU foreign policy chief Catherine Ashton to set a date and venue for a meeting to discuss resuming talks that have been stalled for nearly a year, foreign ministry spokesman Ramin Mehmanparast told reporters Tuesday.
But a spokesman for Ashton shot back that Iran "must first respond" to an October letter from Ashton proposing renewed talks, "and then we'll take it from there."
The negotiations were being held with the five permanent members of the UN Security Council -- Britain, China, France, Russia and the United States -- plus non-permanent member Germany.
The international pressure has already hit Iran's economy, scaring off foreign investors and complicating payments for oil exports.
Officials said recent volatility in Iran's currency, the rial, that worsened after the latest US measures were enacted on the weekend, had nothing to do with sanctions.
The Iranian central bank injected foreign currency into the domestic market on Tuesday, reversing much of a 12-percent fall in the rial on Monday, according to Commerce Minister Mehdi Ghazanfari, quoted by the state IRNA news agency.