TEHRAN (AFP) - A showdown between Iran and the United States over Tehran's threats to close the strategic Strait of Hormuz to oil tankers worsened Thursday with warships from each side giving weight to an increasingly bellicose exchange of words.

Iran's Revolutionary Guards rejected a warning that the US military would "not tolerate" such a closure, saying they would act decisively "to protect our vital interests."

The tough language came as Iran's navy said a US aircraft carrier entered a zone where its ships and aircraft were in the middle of 10 days of war games designed to be a show of military might. The area was in waters to the east of the Strait of Hormuz, a choke point at the entrance to the Gulf through which more than a third of the world's tanker-borne oil passes.

Iranian Vice President Mohammad Reza Rahimi warned this week that "not a drop of oil will pass through the Strait of Hormuz" if the West followed through with planned additional sanctions against Iran over its nuclear programme. The navy commander, Admiral Habibollah Sayari, backed that up by saying it would be "really easy" to close the strait. A US Defence Department spokesman replied with a declaration Wednesday that "interference with the transit... of vessels through the Strait of Hormuz will not be tolerated."

But Brigadier General Hossein Salami, the deputy commander of Iran's powerful Revolutionary Guards, told Fars news agency on Thursday that "our response to threats is threats." "We have no doubt about our being able to carry out defensive strategies to protect our vital interests -- we will act more decisively than ever," he was quoted as saying. "The Americans are not qualified to give us permission" to carry out military strategy, he said.

Admiral Sayari said the US aircraft carrier was monitored by Iranian forces as it passed from the Strait of Hormuz to the Gulf of Oman, according to state television. The network showed footage of an aircraft carrier being followed by an Iranian plane.An Iranian navy spokesman, Commodore Mahmoud Mousavi, told the official IRNA news agency the US carrier went "inside the manoeuvre zone" where Iranian ships were conducting their exercises.

He added that the Iranian navy was "prepared, in accordance with international law, to confront offenders who do not respect our security perimeters during the manoeuvres."

The aircraft carrier was believed to the USS John C. Stennis, one of the American navy's biggest warships.