The military manouevres come after the United States said it was preparing "alternatives" in case negotiations to revive a deal to curb Iran's nuclear programme collapse in Vienna.

"We have carried out exercises to destroy the enemy before they approach the Hormuz islands," Guards navy commander Rear Admiral Alireza Tangsiri said, quoted by the Guards' Sepah News website.

Abu Musa island and the Greater and Lesser Tunb islands, located in the Gulf near the entrance to the Strait of Hormuz, are under Iranian control but are also claimed by the United Arab Emirates.

A fifth of world oil output passes through the Strait of Hormuz.

The military drills dubbed Payambar-e-Azadm, or "Great Prophet", began on Monday in Bushehr, Hormozgan and Khuzestan provinces, each of which touch the Gulf.

They included biological warfare exercises.

The manouevres also saw the deployment of Iranian-made boats that are capable of launching high-precision missiles and reaching speeds up to 75-95 knots.

At dawn on Monday, "in order to increase the defence capability of the armed forces, an exercise was held over the Bushehr nuclear power plant," Mohammad-Taghi Irani, Bushehr's deputy governor for political and security affairs, told Fars news agency.

On December 4, an Iranian air defence test in the Natanz region of central Iran caused an explosion heard about 20 kilometres (12 miles) from the Natanz uranium enrichment facility.