MOSCOW, July 8 (RIA Novosti) - One police officer has been killed and three more wounded in two separate militant attacks in the capital of Russia's North Caucasus Republic of Daghestan, a police source told RIA Novosti on Wednesday.

The source said that unknown suspects opened fire on policemen who had stopped their Lada car to check documents in the Sovetsky district of the republic's capital of Makhachkala.

"One of the police officers died in hospital from gunshot wounds and the other remains hospitalized," he said.

In a separate development late on Tuesday night, unknown suspects driving a Lada car opened fire on a police car near Makhachkala's central square, wounding two officers.

Militant attacks and clashes remain common in Russia's North Caucasus, although the Kremlin campaign to fight separatists and terrorists in neighboring Chechnya has officially ended. Violence often spills over into nearby republics, in particular Ingushetia and Daghestan.