BEIRUT, Lebanon - Turkish warplanes shot down a Russian jet Tuesday that Turkey said violated its airspace on the border with Syria, a major escalation in the Syrian conflict that could further strain relations between Russia and the West.

Russian officials confirmed that a Russian Su-24 attack aircraft was shot down but insisted it had not violated the airspace of Turkey, a member of the NATO alliance.

Later, a Russian rescue helicopter was damaged by a rebel missile in Syria after picking up one of the two pilots who apparently ejected from the fire-engulfed plane, a Syrian activist group reported. A separate video purportedly posted by rebels appeared to show the body of the second pilot.

In Washington, President Obama called for de-escalation but said Turkey had the right to defend its airspace.

In Moscow, Russia's Defense Ministry said Tuesday evening that one of the pilots was apparently killed by ground fire, and that a Russian marine stationed in Syria also died during a search-and-rescue operation to find the pilots. The marine was aboard an Mi-8 helicopter that was forced to land after coming under gunfire, Sergei Rudskoi, an officer on the Russian army's general staff, told Russian journalists. The fate of the second pilot was not addressed.

"During the operation, one of the helicopters, due to gunfire, was damaged and forced to make an emergency landing on neutral territory," Rudskoi said. "One marine was killed." The two fatalities were Russia's first confirmed combat deaths since the beginning of the intervention in Syria almost two months ago.

Rudskoi's description of the incident partially matched a video released by Syrian rebels Tuesday showing a Russian helicopter landing on a hillside before being hit by a TOW antitank missile. Rudskoi did not say that the helicopter was hit by a missile.

"A stab in the back," complained Russian President Vladimir Putin in response to the plane downing.

Turkey's military, however, said the Russian jet was warned multiple times before it was targeted by two F-16 fighter jets in the border zone in western Syria in mountains not far from the Mediterranean coast.

NATO allies with intelligence assets near the area confirmed Turkey's version of events, and rejected Russia's claim that its aircraft was flying over Syria and had not crossed into Turkish airspace, NATO Secretary-General Jens Stoltenberg said after an emergency alliance meeting Tuesday.

"The information we have from other allies is consistent with what we have got from Turkey," Stoltenberg said in Brussels.

He called on Russia to join in fighting "the common enemy" - the Islamic State, also known as ISIS and ISIL - and to stop its airstrikes against "parts of Syria where ISIL is not present."

"This is a serious situation" that calls for prudence and de-escalation, Stoltenberg said. "We have to avoid that situations, incidents, accidents spiral out of control."

Turkey called early Tuesday for the meeting of the North Atlantic Council, NATO's ambassador-level political body in Brussels. Stoltenberg spoke after the meeting, in which the Turks briefed fellow alliance members and they shared information.

Stoltenberg recalled that he and the alliance had previously expressed concern about Russian warplanes flying "close to NATO's borders." He added that "as we have repeatedly made clear, we stand in solidarity with Turkey and support the territorial integrity of our NATO ally."

NATO has not indicated it is prepared to take action in response to the incident. Nor has Turkey asked for a military response under the North Atlantic Treaty's Article Five, which requires the alliance to help defend any member that comes under armed attack.

The downing brings renewed attention to a scenario feared for months by the Pentagon and its partners: a potential conflict arising from overlapping air missions over Syria - with Russia backing the regime of Syrian President Bashar al-Assad, and a U.S.-led coalition conducting airstrikes against the Islamic State.

NATO and Russia have been at odds over a series of flash points since the Cold War - including the NATO-led bombings in Bosnia in the 1990s and NATO support for Ukraine last year against pro-Moscow separatists - but the Syrian conflict has now put the two powers in possibly dangerous proximity.

A top European Union official, Donald Tusk, posted a message on Twitter urging all sides to remain "coolheaded and calm" at a "dangerous moment." In the Turkish capital, Ankara, the country's leaders also met in crisis session. The main Turkish stock index dropped 4.4 percent.

The fallout could complicate a diplomatic push to bring greater international coordination to the fight against the Islamic State. The militant group - with strongholds in Syria and Iraq - has claimed responsibility for the Nov. 13 Paris attacks that claimed at least 130 lives, as well as the Oct. 31 downing of a Russian passenger plane over Egypt's Sinai Peninsula that killed all 224 aboard.

As part of the high-level talks, French President François Hollande held meetings with Obama in Washington to discuss strategies against the Islamic State and parallel efforts to seek a negotiated end to Syria's nearly five-year civil war. Hollande is expected to meet later in the week with Putin and other world leaders.

Last month, the Western alliance decried a "troubling escalation" by Russian forces in Syria and raised concerns about attack missions within sight of NATO borders.

The Su-24 is one of dozens of fixed-wing aircraft flying sorties in Syria as part of Russia's two-month-old bombing campaign, which Moscow says is aimed at crippling the Islamic State.

But Russian attacks have heavily targeted rebel groups - some backed by Turkey and its Western partners - seeking to bring down Assad, who is closely allied with Russia and Iran.

In the Russian resort city of Sochi, Putin said the plane "did not threaten the territory of Turkey," and claimed it was "pursuing operations" against the Islamic State in mountainous areas north of the Syrian port of Latakia.

He called the Turkish response a "stab in the back by the accomplices of terrorists" - an apparent reference to Turkish-supported rebel forces fighting Assad.