Tehran said it will test-fire missiles in the Strait of Hormuz on Saturday, a move likely to stoke tensions with Washington already running high over Iran's threats to close the strategic oil waterway if sanctions are enforced.

"Shorter- and longer-range, ground-to-sea, surface-to-surface and surface-to-air missiles will be tested on Saturday," the ISNA news agency quoted Iran's navy spokesman, Commodore Mahmoud Mousavi, as saying on Friday.

Iran, which has been carrying out war games in the Strait of Hormuz over the past week, has said that "not a drop of oil" would pass through the strait if Western governments follow through with planned additional sanctions over its nuclear programme.

The US State Department said on Thursday that Iran's threat to close the waterway, through which more than a third of the world's tanker-borne oil passes, exhibited "irrational behavior" and "will not be tolerated."

The naval manoeuvres launched by Iran in the strait on December 24 have so far included mine-laying and the use of aerial drones, according to Iranian media.

Analysts and oil market traders have been watching developments in and around the Strait of Hormuz carefully, fearing that the intensifying war of words between arch foes Tehran and Washington could spark open confrontation.

With tensions over the straight simmering, Iran-rival Saudi Arabi announced that it had signed a deal to buy 84 new US fighter jets in order maximise defence capabilities in the oil-rich kingdom.

On Thursday, the US said the $30 billion (23 billion euro) deal sent a "strong message" to the Gulf region, where Riyadh has voiced concerns about Tehran's efforts to boost its influence.

In addition to buying the new aircraft, Saudi Arabia will be able to modernize 70 existing planes while procuring munitions, spare parts, training and maintenance contracts, US officials said.

Two US warships had entered a zone where Iran was conducting its war games, but a US naval spokeswoman said the ships were travelling as part of a pre-planned, routine operation and were not deployed as a show of strength.

"Our interaction with the regular Iranian Navy continues to be within the standards of maritime practice, well-known, routine and professional," Fifth Fleet spokeswoman Lieutenant Rebecca Rebarich said on Thursday.

Iran has dismissed all US threats regarding potential consequences over the strait's closure.

Brigadier General Hossein Salami, the deputy commander of Iran's elite Revolutionary Guards, told the Fars news agency on Thursday that Tehran has "no doubt about our being able to carry out defensive strategies to protect our vital interests."

"The Americans are not qualified to give us permission" to carry out military strategy, he was quoted as saying.

Iran's navy chief, Admiral Habibollah Sayari, backed that up by saying it would be "really easy" to close the strait.