Iran, China and Russia began joint naval drills on Friday in the Indian Ocean and Gulf of Oman, in what Moscow said was an unprecedented exercise in naval cooperation and training.
Waters around Iran have become a focus for international tensions, with the United States exerting pressure for Iranian crude oil sales and other trade ties to be cut off.
"The message of this exercise is peace, friendship and lasting security through cooperation and unity... and its result will be to show that Iran cannot be isolated," Iranian flotilla admiral Gholamreza Tahani said on state television.
The broadcaster said the drills included rescuing ships on fire or vessels under attack by pirates and shooting exercises.
The Gulf of Oman is a particularly sensitive waterway as it connects to the Strait of Hormuz, through which about a fifth of the world's oil passes and which in turn connects to the Gulf.
Washington proposed a U.S.-led naval mission following several attacks in May and June on international merchant vessels, including Saudi tankers, in Gulf waters which the United States blamed on Iran. It denies the accusations.
President Donald Trump last year pulled the United States out of Iran's 2015 nuclear deal with six nations and re-imposed sanctions on the country, crippling its economy.
Tension has risen in the region not only over Iran's disputed nuclear program but also over a September attack on Saudi oil facilities blamed on Iran by the United States and Saudi Arabia. Iran also denies involvement in that.
China said on Thursday it was sending a guided missile destroyer to the four-day drills, which it called a "normal military exchange" between the three armed forces.
"It is not necessarily connected with the regional situation," a Chinese Defense Ministry spokesman said.
China has close diplomatic, trade and energy ties with Iran, which has friendly ties with Russia.
Russia's Defense Ministry said on Friday it had sent three ships from its Baltic Fleet — a frigate, a tanker and a rescue tug boat — to take part in the drills, Red Star, the official newspaper of Russia's armed forces, reported.
The ministry was cited as saying that it was the first time that such drills were being held in such a format.
Both Russia and China also have good relations with Iran's regional rival Saudi Arabia.