The United States reported that in November, Iran sent artillery and tank shells to Russia for use in Ukraine — the US National Security Council spokesman, John Kirby, Politico reports

He said that in return, Russia could provide Iran with fighter jets.

"A close military alliance between Russia and Iran is not only certainly going to make things in Ukraine more difficult but could certainly make the security situation in the Middle East more difficult for our partners and friends there," Kirby added.

The United States believes Russia plans to cooperate with Iran to obtain more military equipment: "Russia has offered unprecedented defense cooperation, including missiles, electronics and air defense."

Iran seeks to acquire attack helicopters, radars, and combat training aircraft.

"We will be watching this very closely to see what actually happens," Kirby said.