Iran's military has started its annual war games less than a month before nuclear talks with the west, according to its state TV.
Reports said navy, ground and air force units were participating in the exercise in a coastal zone in the Gulf of Oman.
This was said to include fighter jets, helicopters, military transport aircraft, submarines and drones.
The games were reported to take place east of the critically important shipping lane – the Strait of Hormuz – and it unclear how long they were expected to last.
It comes in a period of heightened tension between Iran and the United States, in the wake of former president Donald Trump's unilateral withdrawal of America from Iran’s 2015 nuclear deal with world powers.
Dubbed "Zolfaghar-1400", the war games are aimed at "improving readiness in confronting foreign threats and any possible invasion", state TV said.
US officials said last week that Iran had seized a Vietnamese-flagged oil tanker in the Gulf of Oman last month and is still holding the vessel in its port.
Iran offered conflicting accounts of what happened, claiming that elite Revolutionary Guard commandos had thwarted a US seizure of a tanker carrying Iranian oil in the Gulf of Oman and freed the vessel.
It aired dramatic footage on state television but did not further explain the incident.
Under the historic nuclear deal, Iran was prohibited from enriching uranium above 3.67%. Enriched uranium above 90% can be used for nuclear weapons, though Tehran insists its nuclear programme is peaceful.
After months of delays, the European Union, Iran and the US announced last week that indirect talks to resuscitate the deal would resume on November 29 in Vienna.