Russian warplanes hit Pentagon-backed Syrian fighters with a barrage of airstrikes earlier this week, disregarding several warnings from U.S. commanders in what American military officials called the most provocative act since Moscow's air campaign in Syria began last year.
The strikes hit a base near the Jordanian border, far from areas where the Russians were previously active, and targeted U.S.-backed forces battling the Islamic State militants.
No U.S. forces were present in the area, but the U.S. military scrambled fighter jets and used an emergency communications channel set up to avoid air accidents to tell Russian officers to end the strikes, according to the officials, who spoke Friday about the incident after requesting anonymity because they were not authorized to discuss the matter publicly.
The Russian Su-34 fighter-bombers left the area at first, but came back for a second strike after the U.S. F/A-18 fighters went to refuel. The second attack killed several Syrian rebels attempting to provide medical support to the survivors of the initial one, officials said.
"It's an egregious act that must be explained," a U.S. official said. "The Russian government either doesn't have control of its own forces or it was a deliberate provocative act. Either way, we're looking for answers."
The incident came amid calls within the U.S. government for a tougher approach to the Syrian conflict.
On Thursday, an internal State Department memo became public in which 51 diplomats, using the State Department's long-standing dissent process, criticized President Obama's policy and called for U.S. military strikes against forces loyal to Syrian President Bashar Assad, Russia's ally.
State Department spokesman John Kirby on Friday conceded that such a large number of signatures on a dissent memo was "unusual."
"No one's content with the status quo," he said. "Too many people are dying."
Assad's army continues to bomb civilian neighborhoods as the bloody civil war plods toward its fifth year. Russian bombing and intervention by Iranian-backed forces have helped give Assad's embattled government an advantage over rebel groups in recent months.
Nearly all the Russian airstrikes over the past nine months have hit northern Syria, in the region around Aleppo, the country's second-largest city. The Free Syrian Army, a loose grouping that has received aid from Washington and its allies in the Persian Gulf and Turkey, has battled government forces for control of the city.
The U.S. has carried out its own air campaign against Islamic State positions in eastern Syria.
These latest strikes occurred on the other side of the country from the usual Russian operations, around Tanf, a town near where the borders of Jordan, Iraq, and Syria meet.
U.S. officials believe the strikes were launched to pressure the U.S. into working with Russia. Moscow has long wanted the U.S. to combine its air campaign in Syria with Moscow's so they can share intelligence and targeting information.
The Obama administration has rejected that idea because it would put U.S. forces on the same side as Assad. The administration says Assad must leave power, although officials have said he could remain for a period of "managed transition."
The Russian strike hit a small rebel base for staging forces and equipment in a desolate, unpopulated area near the border. About 180 rebels were there as part of the Pentagon's program to train and equip fighters against Islamic State.
When the first strikes hit, the rebels called a U.S. command center in Qatar, where the Pentagon orchestrates the daily air war against Islamic State.
U.S. military commanders called their Russian counterparts on a special hotline set up to ensure the two countries' pilots will not mistakenly run into – or fire upon – one another as they conduct daily bombing runs in the skies above Syria.
Two Navy fighter jets were scrambled from one of two U.S. aircraft carriers stationed in the Mediterranean Sea. The U.S. pilots visually identified the pair of Russian jets,which briefly left the area but returned once the American planes went for additional fuel.
U.S. Defense Secretary Ashton Carter briefly addressed the incident with reporters at the Pentagon, saying the U.S. military was attempting to "clarify the facts" on why the communications channel wasn't "professionally" utilized.
Pentagon officials have said in the past that the U.S. military would defend forces it trained if they were threatened inside Syria by Assad's government.
"This was an attack on forces, first of all, that were fighting ISIL," Carter said, using an acronym for Islamic State. "Obviously that's the first thing that's problematic about this Russian conduct."
Kremlin spokesman Dmitry Peskov appeared to confirm the attack Friday, telling reporters it was difficult to distinguish different rebel groups from the air.
Russian President Vladimir Putin has repeatedly said Moscow's air campaign is aimed at Islamic State. But the strikes, instead, have focused on moderate opposition forces and to some extent on the Nusra Front, a group aligned with Al Qaeda, according to analyses by U.S. intelligence officials. The Russian military has launched a relatively small number of airstrikes against Islamic State, mostly for propaganda purposes, U.S. officials say.
Putin has called for a broad international coalition to fight Islamic State, including the United States and other Western countries already waging airstrikes on the militants. He has insisted, however, that Assad be involved in the campaign and that any foreign interventions be subject to the approval of the Syrian leader, whom he has described as a bulwark against terrorism.
"It's about ensuring Assad's hold on power and weakening his enemies," said Dmitry Gorenburg, a senior research scientist on Russia at CNA, a think tank based in Arlington, Va. The Russians feel that "having accomplished a great deal in the north, there's no reason not to do something similar in the south," he said.