Russia toughened its stance toward Turkey on Wednesday and warned it may be mulling other responses, amid reports that U.S. officials believe one of Russia's attack jets was inside Syria when it was shot down by Turkish fighters.

Russian President Vladimir Putin told reporters he had ordered further reinforcement of air defenses in Syria, and endorsed a warning by the Foreign Ministry for Russians to avoid traveling to Turkey, a popular vacation spot.

"You see, the problem goes beyond the tragedy that happened yesterday. It is a much deeper problem. We see, and not only we, but people all around the world see that Turkey's current government has been following a domestic policy of quite conscious Islamicization throughout the country for a number of years now," Putin said.

"After what happened yesterday, we cannot rule out the possibility of other incidents. If such incidents happen, we will have to respond one way or another," he added.

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