WASHINGTON • Struggling to maintain a steady supply of arms for its war in Ukraine, Moscow is looking to Iran once again to resupply the Russian military with drones and surface-to-surface missiles, according to two officials familiar with the matter.

There is growing U.S. concern that Russia may seek to acquire additional advanced conventional weapons from Iran, according to a National Security Council official who spoke on condition of anonymity to discuss U.S. intelligence. The official said the administration is particularly concerned that Russia may seek to acquire surface-to-surface missiles from Iran.

Separately, a U.N. diplomat said Iran has plans to sell Russia hundreds of missiles and drones in violation of the 2015 Security Council resolution that endorsed the nuclear deal between Tehran and six major powers. Russia, as a signatory of the 2015 agreement, would be undermining the resolution, and a key question is what Russia will be giving Iran in return for the drones and missiles, the diplomat said, speaking on condition of anonymity to discuss sensitive internal information.

The diplomat added that the sales would mark a significant increase in the depth of defense cooperation between Russia and Iran and could have “massive implications for security of the region,” the diplomat said.

Concerns about new weapon sales to Russia come after Iran sold hundreds of attack drones to Russian over the summer. The Biden administration says Russia has also turned to North Korea for artillery as the nine-month war grinds on.

On Wednesday, White House National Security Council spokesman John Kirby told reporters that the U.S. has not yet seen Iranian missiles transferred to Russia. But he underscored the impact that previous arms sales have had in the war, as Russian forces have increasingly targeted Ukraine’s civilian infrastructure with the cold winter months setting in.