BRUSSELS — Following the conclusion of the NATO Defense Ministers' meeting here in Brussels on Thursday, US Defense Secretary Ash Carter strongly condemned Russia's actions in Syria and lamented what he characterized as Moscow's policy of self-isolation.

"Instead of engaging in political transition in Syria, which is needed in this long-suffering country, Russia has chosen to double down on their longstanding relationship with [Syrian President Bashar] Assad, committing additional capabilities and personnel," Carter said.

Carter said Russia was committing a strategic mistake by throwing its weight behind Assad's embattled regime, which has suffered a series of setbacks against various rebel and extremist factions in recent months.

"Russia has continued to wrap itself in a shroud of isolation," the US defense secretary said.

The U.S. Defense Secretary said that While the US will keep the door open for cooperation with Russia if Moscow chooses to change tack in Syria, he said, "we have not and will not agree with Russia so long as they continue to pursue this strategy."

Carter joined NATO officials in condemning Russia's decision yesterday to launch cruise-missile strikes against alleged Islamic State positions in Syria — passing over Iranian airspace — from ships in the Caspian Sea without notifying the international coalition operating in the region.

"We have seen increasingly unprofessional activity on the part of Russian forces," Carter said in reference to the cruise missile strikes. Russian fighters have also made close approaches to a US drone and are providing air support for Assad's land forces, he said.

The US, however, is looking to establish a set of agreed-upon safety measures to ensure there are no incidents between Russian and American fighters operating in Syria, Carter said, without explaining why this has not been established after a week of Russian operations.

Carter said the US will continue its operations with or without Russia's assistance, and concluded his remarks by warning that Russia's actions will have consequences for Russia, "and I suspect in the coming days Russia will suffer casualties in Syria."