The United States said Monday that Iran and Russia were “expanding their unprecedented defense partnership” as Moscow ramps up ties with Tehran after being hit with Western sanctions over the invasion of Ukraine.

“We are using the tools at our disposal to expose and disrupt these activities, and we are prepared to do more,” U.S. National Security Council spokesman John Kirby told reporters.

Kirby’s comments came two months after Iran finalized a deal to buy Sukhoi-35 fighter jets from Russia – an indication of deepening defense cooperation between the two countries.

Tehran has forged strong ties with Moscow in various sectors in the past year. Kyiv has accused Tehran of supplying Moscow with Shahed-136 “kamikaze” drones used in attacks on civilian targets since Russia’s invasion of Ukraine last year – an allegation Iran denies.

The United States has also expressed alarm over the growing military cooperation between Iran and Russia, with Kirby warning in December that Russia looked likely to sell Iran its fighter jets. Kirby maintained that Iranian pilots were reportedly learning to fly the Sukhoi warplanes in Russia and that Tehran may receive the aircraft within the next year, which would “significantly strengthen Iran’s air force relative to its regional neighbors.”

Iran currently has mostly Russian MiG and Sukhoi fighter jets that date back to the Soviet era, as well as some Chinese aircraft. Some U.S.-made fighter jets dating back to before the 1979 Islamic Revolution are also part of its fleet. –i24 News