Russia took new steps to protect its aircraft in Syria on Tuesday as U.S. and NATO officials urged calm over the downing of a Russian attack jet by Turkish fighters amid concern that the incident represents a dangerous escalation in an already complex situation.

Turkish officials said the Russian Su-24 was given 10 warnings in five minutes after it strayed into Turkish airspace along the Syrian border before being shot down by two F-16 fighter jets, the official Anatolia news agency said.

"This incident happened within the framework of the rules of engagement that Turkey had already announced," Turkish President Recep Tayyip Erdogan said. "No one should have any doubt that we have done our best to avoid this incident."

But Russian President Vladimir Putin called the shootdown "a stab in the back."

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