Russia reportedly deployed fighter jets to war-torn Syria to help support the regime of President Bashar al-Assad.
U.S. defense officials later Friday confirmed the move, saying four of the tactical aircraft were observed in the country.
While the Pentagon didn't say what kind of planes were involved, at least one social media report indicated Su-30 aircraft were spotted at Bassel Al-Assad International Airport in Latakia.
The twin-engine fighters carry the NATO designation Flanker-C. The planes are capable of flying air-to-air and air-to-ground missions, and are primarily used by China, India and Vietnam, in addition to Russia.
The deployment of the fighter jets -- coming just hours after Defense Secretary Ash Carter spoke to his Russian counterpart -- is the most visible signal yet that Russia President Vladimir Putin plans to use military force to prop up Assad.
"We've been providing and will be providing all necessary military technical assistance and we call on other countries to join us," Putin said in a televised address.
In an appearance last week with U.S. troops, President Barack Obama last week criticized Putin's support for Assad as "a big mistake."
Russian leaders "are going to have to get smarter," he said. "They are threatened in many ways more than we are" by militants affiliated with the Islamic State in Iraq and Syria, or ISIS, he said. "Their strategy of doubling down on Assad right now is a big mistake."
Yet U.S. lawmakers on Wednesday excoriated Army Gen. Lloyd Austin, head of U.S. Central Command, for his testimony on the progress of the U.S.-led campaign to train moderate rebels in Syria and conduct airstrikes against ISIS.
The $500 million effort begun last December to train and equip Syrian rebels in the campaign against ISIS has produced thus far only four or possibly five fighters actually on the ground in Syria, Austin told the Senate Armed Services Committee.
Sen. John McCain, R-Arizona, called it the worst testimony he had ever heard.
Austin sat stone-faced as McCain called his claims "divorced from reality. Basically, what you're telling us is everything is fine" but "this is an abject failure." In his 28 years in the Senate, "I never heard testimony like this, never."
Some 330,000 people have been killed in the nearly five-year-old Syrian civil war, according to the Syrian Observatory for Human Rights. The conflict has also triggered a refugee crisis in Europe.