“This will have consequences for Russia itself,” Carter said. “And I also expect that in coming days, the Russians will begin to suffer casualties in Syria.”
Syria launched a ground offensive on Wednesday against rebel positions around the Al-Ghab Valley near Hama in central-western Syria, U.S. officials said.
Russian troops near Hama also supported the ground offensive by using howitzer artillery and multiple rocket launch systems to fire at rebel fighters, U.S. officials told ABC News.
It marks the first time since Russian troops began arriving in early September that they have engaged in ground combat in Syria, though they are seen mainly as being in a supporting role to enable the Syrian ground offensive with their precise weaponry.
The U.S. estimates there are now at least 2,000 Russian military personnel at the airbase in Latakia, Syria, from which Russia has launched its air campaign.
“We've seen increasingly unprofessional behavior from Russian forces,” Carter said. “They violated Turkish airspace, which as all of us here made clear earlier this week, and strongly affirmed today here in Brussels, is NATO airspace. They've shot cruise missiles from a ship in the Caspian Sea without warning. They've come within just a few miles of one of our unmanned aerial vehicles. They have initiated a joint ground offensive with the Syrian regime, shattering the facade that they're there to fight ISIL.”
Carter reiterated that Russia has not been targeting ISIL -- also known as ISIS -- as it has claimed, and that the U.S. "will not agree to cooperate with Russia so long as they continue to pursue this misguided strategy.”
“If anything, they're a reminder of erratic and self-defeating behavior and the need for the NATO alliance and its other partners in Europe to stand strong and stand united and stand steady," Carter said.
“It remains our hope that Russia will see that tethering itself to a sinking ship is a losing strategy because Russia has the opportunity to change course and do the right thing,” he added. “I don't know if they will.”
The defense secretary described Russia’s growing isolation from the international community “is a phenomenon that I think Russia's going to have to reckon with," and he cautioned that Russia has developed a pattern of “saying one thing and doing another.”
“We have to watch behavior and not take at face value what Russia says because our experience here -- Ukraine and elsewhere -- is that sometimes, the deeds and the words don't match up,” Carter said.