Russian President Masoud Pezeshkian announced during a visit to Moscow that Russia and Iran agreed to strengthen military cooperation.
The two countries signed a 20-year “strategic partnership” treaty covering areas ranging from defense and technology to energy and trade.
Iran already supplies drones to Russia and, according to Western intelligence, ballistic missiles.
Since launching its full-scale invasion of Ukraine in February 2022, Russia has forged closer ties with Iran, China and North Korea.
In the treaty, Russia and Iran committed to consultations and cooperation to counter military and security threats and to participate in joint military exercises inside and outside the territories of the two countries.
They also agreed not to allow their territories to be used for actions that threaten each other.
Iranian President Pezeshkian said that the signing of the agreement means that relations between Russia and Iran are entering a new chapter.
Trade between the two countries could be significantly enhanced through greater cooperation in the energy sector.
Russian President Vladimir Putin said at a joint press conference with his Iranian counterpart that Russia is considering building new nuclear power units in Iran.
Moscow is also seeking to sell some of its remaining gas to Tehran. The EU has sharply reduced gas imports from Russia since the outbreak of the war in Ukraine, prompting Moscow to sign up new customers.
A plan for a natural gas pipeline from Russia to Iran being negotiated between the two countries would pass through Azerbaijani territory if implemented, the Interfax news agency reported on Friday, citing Russian Energy Minister Sergei Chivilev.
Putin said Russia could eventually supply Iran with up to 55 billion cubic meters of natural gas per year.