GROZNY, April 23 (RIA Novosti) - Gunmen opened fire on an Mi-8 helicopter of the Federal Security Service (FSB) on Wednesday morning in Russia's North Caucasus republic of Chechnya, a law enforcement source said.

However, the republic's interior ministry said it had received no information about the incident.

The law enforcement source told RIA Novosti that the aircraft was damaged and had to make an emergency landing, but that no casualties have been reported.

He said an investigation and search for the gunmen is underway.

Although the active phase of the antiterrorism campaign in the North Caucasus officially ended in 2001, periodic bombings and clashes between militants and federal troops still disrupt Chechnya and nearby regions, including Daghestan and Ingushetia.