The United States on Friday expressed alarm over a "full-scale defense partnership" between Russia and Iran, describing it as "harmful" to Ukraine, Iran's neighbors and the world.

Iran stands accused by Western powers of supplying drones to Russia for its war against Ukraine, as Moscow batters the country's energy infrastructure in search of an advantage in the bloody conflict.

Washington has previously condemned Iran-Russia security cooperation, but on Friday described an extensive relationship involving equipment such as drones, helicopters and fighter jets.

"Russia is seeking to collaborate with Iran in areas like weapons development, training," White House national security spokesman John Kirby told reporters.

Moscow "is offering Iran an unprecedented level of military and technical support -- that is transforming their relationship into a fully fledged defense partnership," he said.

"We have also seen reports that Moscow and Tehran are considering the establishment of a joint production line for lethal drones in Russia. We urge Iran to reverse course (and) not to take these steps."

Kirby said that the United States would sanction three Russian-based entities active in "the acquisition and use of Iranian drones."

Last month, Tehran admitted it had sent drones to Russia, but insisted they were supplied before the invasion of Ukraine.

Kirby said the United States is also concerned that Russia "intends to provide Iran with advanced military components," including helicopters and air defense systems.

- 'Sordid deals' -

Iranian pilots have reportedly been learning to fly advanced Sukhoi Su-35 warplanes in Russia, and Tehran may receive the aircraft within the next year, which would "significantly strengthen Iran's air force relative to its regional neighbors," Kirby said.

RTL

A Russian Su-35 warplane, which the White House says Moscow may be training Iranian pilots to fly / © AFP/File

The United States also believes that Iran is considering the sale of "hundreds of ballistic missiles" to Russia, he said.

Britain's Foreign Secretary James Cleverly took aim Friday at the "sordid deals" between Moscow and Tehran, saying in a statement that Iran had sent drones to Russia in exchange for "military and technical support" from Moscow.

This "will increase the risk it poses to our partners in the Middle East and to international security," Cleverly said, vowing that "the UK will continue to expose this desperate alliance and hold both countries to account."

Kirby on Friday announced a new $275 million aid package to help boost Ukraine's air defenses, against Russian drones in particular.

He said the aid "will soon be on its way to provide Ukraine with new capabilities to boost its air defenses and counter the threats that Ukraine is facing from drones."

The Pentagon released details on the package, saying it includes counter-drone equipment, as well as ammunition for Himars precision rocket systems, 80,000 155mm artillery rounds, some 150 generators, and other equipment.

The United States has previously said that generators were being provided to Kyiv to help Ukraine with its electricity needs amid repeated Russian strikes on energy infrastructure.

The latest package -- which is made up of equipment taken from existing US stocks -- brings Washington's military assistance to Ukraine since Russia's February 24 invasion to more than $19.3 billion.