Iran has started six days of naval drills in the Strait of Hormuz aimed at showcasing its military capabilities in what is a vital oil and gas shipping route, the official IRNA news agency reported.
The manoeuvres began early on Friday, involving warships, submarines jet fighters and hovercrafts.
Naval commander Habibollah Sayyari said on Friday the "Velayat 91" drills would last until Wednesday across an area of about one million square km in the Strait of Hormuz, the Gulf of Oman and northern parts of the Indian Ocean, IRNA said.
Sayyari said the goal of the manoeuvres were to show "the armed forces' military capabilities" in defending Iran's borders.
Iran's state TV reported that Tehran had warned ships to stay away from the site until January 3.
The drills come as the West increases its pressure over Iran's nuclear programme. The West suspects it may be aimed at producing nuclear weapons, a charge Iran denies.
Iranian officials have often said Iran could block the strait - through which 40 percent of the world's sea-borne oil
exports pass - if it came under military attack over its disputed nuclear programme.
Iran held a similar 10-day drill last December and sent a submarine and a destroyer into the Gulf four months ago just as US and allied navies were conducting exercises in the same waters to practice keeping oil shipping lanes open.
Sayyari was quoted as saying on Tuesday that the new drill would test the navy's missile systems, combat ships, submarines and patrol and reconnaissance methods.
A heavy Western naval presence in the Gulf is meant to deter any attempt to block the waterway.