MAKHACHKALA, August 18 (RIA Novosti) - Four people were injured in an explosion in Dagestan early on Tuesday, the volatile southern Russian republic's interior ministry said.

"An unidentified explosive device went off at 10:05 local time (06:05 GMT) not far from the market in Kizilyurt [a town west of the republic's capital Makhachkala]. The blast went off when a police patrol was passing by. Four people, including two police officers, were injured," a ministry official said.

He said investigation was underway.

Russia's mainly Muslim North Caucasus republics have been swept by a wave of violence in recent months, with attacks on troops, police and other officials a daily occurrence.

Neighboring Ingushetia saw its deadliest attack in years on Monday. A suicide bomber rammed an explosives-laden minivan into the gates of the police headquarters in the republic's largest city Nazran, killing at least 20 people and injuring 138, mainly civilians.

President Dmitry Medvedev dismissed Ingushetia's interior minister on Monday and ordered a probe into police performance in the wake of the bombing.