DUSHANBE (Reuters) - Tajikistan's security services are investigating unspecified threats of action this month purportedly made by a former commander of the Central Asian nation's elite police force who has defected to Islamic State, security sources said on Tuesday.

Colonel Gulmurod Khalimov, who has gone through special forces training in Russia and the United States, joined the militant group last May.

Last week, Washington offered a reward of up to $3 million for information leading to the location, arrest, and conviction of Khalimov, describing him as "a key leader" of the Syria- and Iraq-based group.

Two security sources told Reuters that this week servicemen in Tajkikistan, a country bordering Afghanistan and seen by the West and Russia as a possible conduit both for militant Islamists and drug runners, started receiving text messages on their mobile phones in Khalimov's name.

In these messages, he promised to "congratulate" them on the 25th anniversary of independence celebrated on Sept.9, the sources said. Security services were investigating the matter.

The evident threats come on the first anniversary of an attempted coup staged by then deputy defense minister, General Abdukhalim Nazarzoda. He died fighting pro-government troops shortly afterwards.

President Imomali Rakhmon's government accused local Islamists, who had their own, legitimate political party, of being behind the coup. Tajikistan's Supreme Court has since outlawed the party while its leaders were sentenced to lengthy prison terms.

(Reporting by Nazarali Pirnazarov; Writing by Olzhas Auyezov; editing by Ralph Boulton)