U.S. and Russian fighter jets bloodlessly tangled in the air over Syria on June 16 as the American pilots tried and failed to stop the Russians from bombing U.S.-backed rebels in southern Syria near the border with Jordan.

The aerial close encounter underscores just how chaotic Syria’s skies have become as Russia and the U.S.-led coalition work at cross-purposes, each dropping bombs in support of separate factions in the five-year-old civil war.

The near-clash also highlights the escalating risk of American and Russian forces actually coming to blows over Syria, potentially sparking a much wider conflict between the world’s leading nuclear powers.The incident began when at least two twin-engine Su-34 bombers, some of Moscow’s most advanced warplanes, struck what the Pentagon described as a “border garrison” housing around 200 U.S.-supported rebels in At Tanf on the Syrian side of the Syria-Jordan border.The rebels had been “conducting counter-ISIL operations in the area,” the Pentagon stated on June 18, using an alternative acronym for ISIS.The United States and its allies in Syria clearly did not expect the air strike. The rebels in At Tanf are party to a shaky ceasefire agreement between rebel forces and the regime of Syrian president Bashar Al Assad—and, by extension, the Russian military contingent backing Al Assad. The Los Angeles Times reported that Russian planes had not previously been active over At Tanf.

The Su-34s’ initial strike wounded, and perhaps killed, some of the rebels in At Tanf.

The U.S. Navy scrambled F/A-18 fighters to intercept the Russians, the Los Angeles Times reported. The Navy has deployed two aircraft carriers to the region for strikes on ISIS. As the F/A-18s approached the Su-34s, officials with U.S. Central Command—which oversees America’s wars in the Middle East and Afghanistan—used a special hotline to contact their Russian counterparts directing Russia’s own intervention in Syria.

Arriving over At Tanf, the American pilots apparently spoke directly to the Russian aviators. “Pilots CAN communicate with one another on a communications channel set up to avoid air accidents,” Central Command confirmed in a statement to The Daily Beast.

Washington and Moscow had established the hotline as part of a so-called Safety of Flight Memorandum of Understanding that the two governments signed in October specifically in order to avoid the kind of aerial confrontation that occurred over Syria last week.

With the American jets flying close enough to visually identify the Su-34s, the Russians departed the air space over At Tanf. Some time shortly thereafter, the F/A-18s ran low on fuel and left the area in order to link up with an aerial tanker. That’s when the Su-34s reportedly returned to At Tanf—and bombed the rebels again.

According to the Los Angeles Times, the second strike killed first-responders assisting survivors of the first bombing run.

The next day, senior U.S. Defense Department officials organized an “extraordinary” video conference with Russian counterparts to discuss the incident. The meeting included Acting Assistant Secretary for International Security Affairs Elissa Slotkin and U.S. Marine Corps Lt. Gen. Kenneth McKenzie, a strategic planner on the Pentagon’s joint staff, plus unspecified Russian Ministry of Defense officials.

“Department officials expressed strong concerns about the attack on the coalition-supported counter-ISIL forces at the At Tanf garrison, which included forces that are participants in the cessation of hostilities in Syria, and emphasized that those concerns would be addressed through ongoing diplomatic discussions on the cessation of hostilities,” Defense Department spokesman Peter Cook explained in a statement.

"Regarding safety, department officials conveyed that Russia’s continued strikes at At Tanf, even after U.S. attempts to inform Russian forces through proper channels of on-going coalition air support to the counter-ISIL forces, created safety concerns for U.S. and coalition forces,” Cook continued. “Department officials requested Russian responses to address those concerns.”

Russian Defense Ministry spokesman Igor Konashenkov confirmed, via the country’s state-owned media, that the teleconference took place—but he did not specify the results of the “extraordinary” meeting.

Russian warplanes had previously shadowed planes belonging to the U.S.-led coalition over Syria, but the coalition always described the Russians’ behavior as “professional.” By contrast, in April Russian Su-24 bombers repeatedly buzzed the U.S. Navy warship USS Donald Cook while the vessel sailed in international waters in the Black Sea. A Pentagon spokesman called the Russians’ actions in that incident “provocative and unprofessional.”

The Kremlin should be keenly aware of the potential for unwanted—and potentially destabilizing—bloodshed that exists in the air over Syria. In November, a Russian Su-24 bomber flying a mission over Syria strayed over the Syria-Turkey border into Turkey—and a Turkish F-16 fighter promptly shot it down.

The two Russian crew members ejected. One flier died when Syrian rebels on the ground opened fire on his parachute. Russian, Syrian, and Iranian forces launched a complex rescue mission that ultimately retrieved the surviving pilot. One Russian marine died and a helicopter was destroyed during that operation.

The fallout from the November incident continues, with Russia and Turkey exchanging threats—and Moscow imposing economic sanctions on Ankarra including limits on some food imports to Russia from Turkey.

It’s not clear how close the U.S. fighters came to attacking and potentially shooting down the Su-34s over At Tanf. Central Command declined to say what the rules of engagement are for American pilots flying over Syria. “ROE are actually specifics that we don’t get into,” Central Command said in a statement.

The last time a U.S. military warplane shot down a Russian—actually, Soviet—plane was in 1953, over Korea or China, depending on which historians you believe. The last time a Russian or Soviet warplane shot down an American aircraft was in 1970, when a U.S. Army plane strayed over Armenia.