The "Aras-2025" exercise, running through May 21, involves Iran's Islamic Revolutionary Guard Corps and Azerbaijani troops, the IRNA news agency reported.

The drill was being held in areas of Karabakh previously disputed with Armenia before Azerbaijan regained control in September 2023.

"This drill is a key step in boosting border security and confronting potential threats," said Brigadier General Vali Madani of the Guards' ground forces.

Last month, Iranian President Masoud Pezeshkian made a rare visit to Baku, where he met his Azerbaijani counterpart Ilham Aliyev.

"Iran will make efforts to ensure that its relations with Azerbaijan are strategic across all spheres," Pezeshkian said at the time.

Ties between the two neighbours were strained over Azerbaijan's close security cooperation with Israel and a deadly 2023 attack on its embassy in Tehran.

Baku reopened the embassy in mid-2024 and the attacker was sentenced to death.

Tehran has been strongly opposed to the so-called Zangezur corridor linking Azerbaijan to Turkey which would run along Iran's border with Armenia.

In November, the two countries held a joint naval exercise in the Caspian Sea.