"They will not close it… They will not be allowed to close it," Vice-Admiral Kevin J. Cosgriff told a press conference in Bahrain, where the Fifth Fleet is based.
His remarks followed comments by the chief of Iran's elite Revolutionary Guards, General Mohammad Ali Jafari, who issued a new warning last week against any attack against his country over its controversial nuclear drive.
"It is natural that when a country is attacked it uses all of its capabilities against the enemy, and definitely our control of the Persian Gulf and the Strait of Hormuz would be one of our actions," Jafari said.
The strait between Iran and Oman is a vital conduit for energy supplies, with as much as 40 percent of the world's crude passing through the waterway from Gulf suppliers.
"Certainly if there is fighting… the scope will be extended to oil, meaning its price will increase drastically. This will deter our enemies from taking action against Iran," Jafari said.
Cosgriff said: "The latest Iranian statements are not helpful."
He insisted that that the international community will work to protect navigation in the Strait of Hormuz, adding that any action by Iran "will not be an action against the United States but against the international community".
According to news reports, more than 100 Israeli warplanes staged a training exercise with Greece earlier this month to prepare for a possible long-distance strike and as a warning to Tehran.
But Cosgriff said he did not see "any reason for Israel to strike Iran" in the short term.
Iran has been slapped with three sets of UN sanctions over its defiance of Security Council ultimatums to suspend uranium enrichment, the process which produces nuclear fuel for civilian reactors but in highly extended form can also make the fissile core for an atomic bomb.
Iran insists its nuclear ambitions extend only to generating electricity for a growing population but both Israel and the United States suspect it of trying to develop a bomb.
There have been several confrontations between Iranian and US vessels in the Gulf this year.
Iran, the OPEC oil cartel's number two producer, has said that using oil as weapon is not on its agenda — but has also not ruled it out.
A former head of Israel's Mossad foreign intelligence agency, Shabtai Shavit, said in comments published on Sunday that the Jewish state had one year to destroy Iran's nuclear programme or face the risk of coming under nuclear attack.
Israel has the Middle East's sole if undeclared nuclear arsenal.
