Turkish military aircraft shot down a Russian fighter jet Tuesday because they claim it violated its airspace on the Syrian border, injuring the pilots and maybe more.

Already Russian President Vladimir Putin has called the action “a stab in the back”.

Russia has confirmed that the jet — a Su-24 Fencer — was shot down, but refuses to say that it violated Turkey’s airspace.

Turkey, a member of the NATO alliance, insists that the Russian plane was warned numerous times before it was engaged and destroyed over mountains close to the Mediterranean coast.

Soon after, a Russian rescue helicopter was damaged when a Syrian rebel missile hit it upon picking up the two pilots of the Fencer.

Broadly, it’s possibly the most extreme incident to arise from the “overlapping air missions” over Syria that have seen Russia backing Syrian President Bashar al-Assad in their assault and the U.S.-led coalition airstrikes which have targeted ISIS.

This from the Washington Post:

In further signs of the spiraling tensions, NATO called an emergency meeting. But Turkey has not invoked Article Five, which requires NATO members to aid any that comes under armed attack, a NATO official said.

A top European Union official, Donald Tusk, posted a Twitter messaging urging all sides to remain “cool-headed and calm” at a “dangerous moment.” In the Turkish capital Ankara, the country’s leaders also met in crisis session.

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.