Prime Minister Ahmet Davutoğlu speaks at a press conference about Turkey’s right to respond to airspace violations

Turkey shot down a Russian warplane near the Syrian border on Tuesday, saying the jet had violated its airspace. It is one of the most serious publicly acknowledged clashes between a NATO member country and Russia for half a century.

Russian President Vladimir Putin said the plane had been attacked when it was less than a quarter-mile inside Syria. He warned of “serious consequences” for what he termed a stab in the back administered by “the accomplices of terrorists”.

“We will never tolerate such crimes like the one committed today,” Putin said, as Russian and Turkish shares fell on fears of an escalation between the former Cold War enemies.

In a letter to the U.N. Security Council, Turkey said it had shot down the jet while in Turkish airspace. Along with a second plane, the aircraft had flown more than a mile into Turkey for 17 seconds, despite being warned 10 times in five minutes while approaching to change direction, the letter said.

“Nobody should doubt that we made our best efforts to avoid this latest incident. But everyone should respect the right of Turkey to defend its borders,” Turkish President Tayyip Erdogan said in a speech in Ankara.

In condemnation of Russian air strikes in Syria, during which Turkish airspace has been violated several times in recent weeks, Erdogan said that only Turkey’s “cool-headedness” had prevented worse incidents in the past.

Each country summoned a diplomatic representative of the other. Russia’s Foreign Minister Sergei Lavrov canceled a visit to Turkey due on Wednesday and the defense ministry said it was preparing measures to respond to such incidents.

President Obama and French President Francois Hollande, meeting in Washington, urged against an escalation, while NATO Secretary-General Jens Stoltenberg said the military alliance stood in solidarity with Turkey.

Footage from private Turkish broadcaster Haberturk TV showed the warplane going down in flames, a long plume of smoke trailing behind it as it crashed in a wooded part of an area the TV said was known by Turks as “Turkmen Mountain”.

Separate footage from Turkey’s Anadolu Agency showed two pilots parachuting out of the jet before it crashed.

A deputy commander of rebel Turkmen forces in Syria said his men shot both pilots dead as they came down. The Russian military confirmed one pilot had been shot dead from the ground and another soldier died during a rescue operation.

A senior Turkish official said at least one of the pilots could still be alive. “It’s not a fact but a possibility. We’re trying to verify the information and taking all necessary steps to facilitate their return,” the official said.