MOSCOW — A senior police commander from Tajikistan who vanished last month has reappeared in an online video, saying he had defected to join the Islamic State group in Syria.

The development on Thursday raised concerns of growing extremism in Tajikistan, a former Soviet republic that Russia supports as a bulwark against militant Islamism.

The officer, Col. Gulmurod Khalimov, was the commander of the elite OMON division of the Tajik national police. In the video, shot in high definition at an unnamed location, the colonel is seen declaring his intention to wage jihad in Central Asia.

Wearing a black turban and holding a sniper rifle, Colonel Khalimov says in the video that he traveled to the United States three times and received training there from Blackwater, the security company that was once a major contractor for the American government in Iraq and elsewhere. (The company is now known as Academi.)