The United States believes Russia and Iran are expanding an unprecedented defense partnership that will help Moscow prolong its war in Ukraine, the White House said Monday.

As part of the cooperation, Iran is providing Russia with one-way attack drones, including more than 400 since August, national security adviser John Kirby said at a news briefing.

"This is about a burgeoning defense relationship" that allows Russia to kill more people in Ukraine and also enables Iran to stock up on its military hardware and pose a greater threat to its neighbors, Kirby told reporters.

The US and its European allies have already imposed sanctions on individuals and entities for their role in Tehran’s drone program and their delivery to Russia. Washington has also listed Tehran’s military cooperation with Moscow as one of the impediments to resuming talks aimed at limiting Iran’s nuclear program and lifting crippling economic sanctions.

Drones are the primary military help Iran is providing to Russia, which is seeking to acquire advanced level types, he said.

Russia uses Iranian Shahed drones mainly to target civilian infrastructure and also swarm Ukrainian air defenses during large-scale missile strikes. The tactic was repeatedly used last year when Moscow tried to cripple Ukraine’s energy sector.

The Unites States will be announcing additional designations targeting increased military cooperation between Russia and Iran, Kirby added.