Turkish armed forces have shot down a Russian military aircraft near its border with Syria.
The Russian defence ministry confirmed that one of its planes crashed, but insisted that it never crossed Syrian airspace.
Video footage emerging from the region appeared to show a jet crashing into hills on the Syrian side of the border. This has not yet been verified:
The fate of the pilots is unclear, amid reports they ejected from the jet.
A spokesperson for an insurgent group in Syria told the Associated Press its forces fired at the pilots after they ejected
What does this all mean?
Under Article 5 of the Nato alliance, which incorporates 28 nations including Britain and the US, all member states are required to come to the aid of any other member which comes under attack.
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Nato Article 5
Following the crash on Tuesday morning, a Nato official confirmed that the Turkish government will be informing the alliance of what happened in the afternoon.
There will be a North Atlantic Council meeting this afternoon at 5 (1600 GMT).
The aim is for Turkey to inform allies about the events of this morning.
Nato spokeswoman Irina Novakova
Has this happened before?
Russian war planes have got close to Nato airspace on a number of occasions over the summer, including into a "UK area of interest".
However, an escalation such as Tuesday's events has not occurred since the end of the Cold War - this is thought to be the first time a Russian plane has been shot down by a Nato member since the 1950s.
Last month, Turkish jets shot down an unidentified drone that it said had violated Turkey's airspace, according to the Associated Press.
Russia has been carrying out air strikes in Syria against anti-regime targets but Turkey had warned it last week that any violation of its airspace could result in "retaliation".
Reza Afshar, a former Foreign Office official and current advisor on Britain's policy in Syria, has allayed fears that the conflict could escalate:
What have Russia and Turkey said?
A Russian defence ministry spokesperson was quoted by AP as saying:
We are looking into the circumstances of the crash of the Russian jet. The Ministry of Defence would like to stress that the plane was over the Syrian territory throughout the flight.
On Russian state television, Vladimir Putin, the president, later branded the attack a "stab in the back by the accomplices of terrorists".
Today’s tragic event will have significant consequences, including for Russia-Turkish relations.
Turkey claimed that it had shot down a Russian war plane that had violated Turkish airspace and ignored 10 warnings within the space of five minutes.
Prime minister Ahmet Davutoglu said on Tuesday that Turkey had the right to respond if its airspace is violated.
In a speech in the capital Ankara, according to Reuters, Davutoglu said the world should know that Turkey would do "whatever is necessary" to guarantee the country's security.