The U.S. and U.K. are increasingly concerned that Russia is sharing secret information and technology with Iran that could bring it closer to being able to build nuclear weapons, in exchange for Tehran providing Moscow with ballistic missiles for its war in Ukraine.

The Kremlin has increased its cooperation with Iran over its ambitions to obtain atomic weapons in recent months, according to Western officials familiar with the matter who spoke on the condition of anonymity to discuss assessments that haven’t been made public.

The development was discussed by U.S. and U.K. officials in Washington this week, the people added, as British Prime Minister Keir Starmer met President Joe Biden at the White House for a strategic meeting on foreign policy. They described it as worrying, and an escalation of Russia and Iran’s military ties.