Russia’s top security official, Sergei Shoigu, arrived in Iran on Tuesday for talks with his counterpart, just a day after reports emerged suggesting growing concerns from the U.S. and U.K. over a potential nuclear deal between the two countries.

Details surrounding Shoigu’s meeting in Iran remain limited, but U.S. officials have expressed increasing worry about the deepening relationship between Iran and Russia amidst the war in Ukraine, suggesting it has reached a troubling new level.

Secretary of State Antony Blinken, during a recent visit to the U.K., confirmed reports that Iran had provided Russia with short-range ballistic missiles to support its ongoing war effort in Ukraine.

However, in comments that largely went unnoticed due to the confirmation of ballistic missile transfers, Blinken also stated, “Russia is sharing technology that Iran seeks – this is a two-way street – including on nuclear issues as well as some space information.”

A report on Monday indicated that President Biden and U.K. Prime Minister Kier Starmer allegedly discussed the possibility of a secret deal being forged, in which Russia has agreed to provide Iran with the technological expertise necessary for developing nuclear weapons.

Nuclear experts, including the U.N.’s International Atomic Energy Agency (IAEA), have warned that Tehran has continued to accelerate its nuclear program for the past three and a half years.

Iran is said to have expanded its stockpiles of highly enriched uranium to levels of 60% purity, falling just short of weapons-grade uranium, which requires a purity level of 90%.

While information about Shoigu’s meeting on Tuesday remains unknown, his trip follows his recent visit to North Korea, another nuclear-armed nation, where he met with leader Kim Jong Un in Pyongyang.

Details of that trip also remain unclear, but it is believed to have served as an opportunity to solidify the Russia-North Korea partnership following the signing of a mutual defense treaty earlier this year.

Western nations have accused North Korea of supplying Russia with arms to aid its war effort in Ukraine, and there are growing concerns that Pyongyang might escalate its military assistance to Moscow.

The U.S. and its Western allies have pledged to hold nations accountable for assisting Russia in its illegal war in Ukraine, but Shoigu’s visits to these adversarial nations coincide with threats issued by Putin towards Washington last week.

Neither the U.S. nor the U.K. have lifted their strike bans on Ukrainian-supplied long-range weapons, which would enable Kyiv to strike deep into Russia – a move they argue is crucial for ending the war with Moscow.

However, Putin last week warned that any attempt by the U.S. and its NATO allies to reverse these strike bans would be considered an act of “war” with Russia, potentially escalating the threat of a Russian strike outside of Ukraine.

Putin has made these threats against the West before, although no strike ban reversals were announced during the recent meetings between Biden and Starmer.

Reuters contributed to this report.