CIA Director Bill Burns warned on Thursday that further growth in the Iranian-Russian defense partnership could pose dangers to the U.S.’ partners in the Middle East.

Speaking at the Aspen Security Forum, Burns addressed the security cooperation between Iran and Russia, including Iran’s provision of weaponized drones that Russia has used in its invasion of Ukraine.

In exchange, Russia has considered providing advanced combat aircraft to Iran, a move Burns said would be a particularly worrisome development that could create further destruction beyond the Ukrainian civilians impacted by the drone attacks.

“The provision of advanced combat aircraft obviously creates threats to our friends in the region as well,” Burns said. 

He described the relationship between Beijing and Moscow as “a useful two-way street” for both parties, noting that Russia has also sent technicians to work on Iran’s space launch and other missile programs.

But Burns also said that the partnership has also not been without its hurdles within Tehran.

“We have… seen signs — and some of this has been public — where the Iranian leadership has hesitated about supplying ballistic missiles to the Russians, which was also on their wish list as well, partly because they’re concerned not just about our reaction, but about European reaction as well,” Burns said.

MI6 Chief Richard Moore said this week that the drone sales had also provoked “internal quarrels at the highest level of the regime in Tehran.” Burns declined to comment specifically on Moore’s report.

Burns added that he’d seen firsthand the impact of the Iranian drones being used “indiscriminately against Ukrainian civilians” during his frequent visits to Kyiv.