The United States warned on Friday that Russia and Iran are ramping up military cooperation to an extent that is "harmful" to Ukraine and Iran's neighbors.

Citing intelligence assessments, White House National Security Council spokesman John Kirby described "an unprecedented level of military and technical support that is transforming their relationship into a full-fledged defense partnership."

Kirby said Iranian pilots are being trained to fly Russian Sukhoi Su-35 fighter jets, which are expected to be delivered next year. This would "significantly strengthen Iran's air force relative to its regional neighbors," he said.

Meanwhile, Kirby said Iran is reportedly considering setting up a jointly-run drone production line in Russia. Iran has previously been accused of supplying Russia with lethal drones for its invasion of Ukraine.

The claims were backed by British Foreign Secretary James Cleverly, who described the defense collaboration and arms transfers as "sordid deals."

Moscow denies defense partnership

Moscow's UN envoy Vassily Nebenzia denounced the US intelligence assessment later on Friday.

"The military industrial complex in Russia can work perfectly fine and doesn't need anyone's assistance, whereas the Ukrainian military industry does not basically exist and is being assisted by the Western industry and Western companies," Nebenzia told the UN Security Council.

He also claimed the allegations about Iranian drones were "already refuted" multiple times before.

Iran has acknowledged that it sold drones to Russia in the past but denies any arms transfers since the invasion of Ukraine.

zc/wd (AFP, AP)