A senior US government official says he is worried that Iran may be thinking about providing Russia with ballistic missiles that could be used in Moscow's invasion of Ukraine.

White House National Security Council spokesperson John Kirby told reporters on Tuesday that Iran has been providing Russia with significant numbers of unmanned aerial vehicles and artillery ammunition. He added that Russia has been using them to attack Ukraine.

He said, "Iran may be prepared to go a step further in its support for Russia."

Kirby mentioned that Iran showed Russian defense minister Sergei Shoigu some of its short-range ballistic missiles, when it hosted him in September.

Kirby said, "In return for that support, Russia has been offering Tehran unprecedented defense cooperation." He noted that includes offers to provide missiles, electronics and air defense systems.

He added that Iran wants to purchase additional military equipment from Russia, such as attack helicopters and radars.

Kirby said the US government has information indicating that the Russian mercenary group Wagner is preparing to provide either Hezbollah or Iran with an air defense capability, at Moscow's direction. Hezbollah is a Lebanon-based Shia Islamic group backed by Iran.

Kirby said the expanding military partnership between Russia and Iran is "obviously harmful" to Ukraine, Iran's neighbors and the international community.