They did not offer any details about how many weapons have been delivered or when the transfers may have occurred, but they confirmed the U.S. intelligence finding. They spoke on condition of anonymity to discuss a matter that has not been publicly disclosed.

The White House declined to confirm the weapons transfer but reiterated its concern that Iran is deepening its support of Russia. The White House has been warning Iran for months not to transfer ballistic missiles to Russia.

“Any transfer of Iranian ballistic missiles to Russia would represent a dramatic escalation in Iran’s support for Russia’s war of aggression against Ukraine and lead to the killing of more Ukrainian civilians,” National Security Council spokesman Sean Savett said in a statement. “This partnership threatens European security and illustrates how Iran’s destabilizing influence reaches beyond the Middle East and around the world.”

The U.S. finding comes as the Kremlin tries to repel Ukraine’s surprise offensive that has led to the seizure of about 500 square miles (1,300 kilometers) of Russia’s Kursk region. Meanwhile, Ukraine’s president, Volodymyr Zelenskyy, is pressing allies to allow his country to use Western-supplied missiles to strike deep inside Russia and hit sites from which Moscow launches aerial attacks.

Iran, as it has with previous U.S. intelligence findings, denied providing Russia with weapons for its war in Ukraine.

“Iran considers the provision of military assistance to the parties engaged in the conflict — which leads to increased human casualties, destruction of infrastructure, and a distancing from ceasefire negotiations — to be inhumane,” according to a statement from Iran’s mission to the United Nations. “Thus, not only does Iran abstain from engaging in such actions itself, but it also calls upon other countries to cease the supply of weapons to the sides involved in the conflict.”

CIA Director William Burns, who was in London on Saturday for a joint appearance with his British intelligence counterpart, warned of the growing and “troubling” defense relationship involving Russia, China, Iran and North Korea that he said threatens both Ukraine and Western allies in the Middle East.

The White House has repeatedly declassified and publicized intelligence findings that show North Korea has sent ammunition and missiles to Russia to use against Ukraine, while Iran supplies Moscow with attack drones and has assisted the Kremlin with building a drone-manufacturing factory.

China has held back from providing Russians with weaponry but has surged sales to Russia of machine tools, microelectronics and other technology that Moscow in turn is using to produce missiles, tanks, aircraft and other weaponry, according to U.S. officials.

The White House has been on edge for months about a possible deal between Iran and Russia.

The Democratic administration said in January that U.S. intelligence officials had determined a Russian-Iran deal had not been completed, but officials were concerned that Russia’s negotiations to acquire missiles from Iran were actively advancing.

Last September, according to the White House, Iran hosted a top Russian defense official to show off a range of ballistic missile systems, adding to the U.S. concern that a deal could come together.

The U.S. and other countries have taken steps aimed at thwarting the supply, sale or transfer involving Iran and ballistic missile-related items, including issuing guidance to private companies about Iranian missile procurement practices to make sure those companies are not inadvertently supporting Iran’s development efforts.

President Joe Biden is set to host British Prime Minister Keir Starmer for talks at the White House on Friday. White House press secretary Karine Jean-Pierre said “continuing robust support to Ukraine in its defense against Russian aggression” will be on their agenda.

The Wall Street Journal first reported on the U.S. intelligence finding.