MOSCOW (Reuters) - Russia has imposed a "counter-terrorism operation" regime in the southern region of Dagestan, detaining three people by Sunday morning, Russian news agencies reported, citing the National Anti-Terrorism Committee.

       Russia is on high alert following a mass shooting at a concert hall in Moscow on March 22 - the deadliest attack in the country in 20 years with at least 144 killed.

       "Security agencies detained three bandits who were planning a number of terrorist offences. During the inspection of the places where the criminals were detained, automatic weapons, ammunition and an improvised explosive device ready for use were found," the committee said on Sunday.

       Earlier the committee said that suspected criminals had been blocked by security services in several flats in residential areas of the regional capital Makhachkala and one of the biggest cities in the republic - Kaspiysk.

       There were no civilian casualties and no losses among the law enforcement personnel.

       The operation continues, the committee said.

       (Writing by Maxim Rodionov; editing by Guy Faulconbridge)

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