MOSCOW, Nov 23 (Reuters) - Four people were killed by a bomb explosion in Russia's volatile Chechen region in the latest attack against security forces in the North Caucasus, local prosecutors said on Sunday.

Chechnya was engulfed by two wars since the break-up of the Soviet Union but has returned to relative calm in the last four years. Violence has become more frequent in nearby regions, notably Ingushetia, in 2008.

In the latest attack, an explosion went off near an apartment building after police went to investigate an earlier report of gunfire around midnight near the capital Grozny.

"Three people, who were police officers, and one passerby, who was a civilian, died in the explosion," the prosecutor said.

On Nov. 6, a suspected suicide bomber killed 12 people at a market in Vladikavkaz in another neighbouring region, North Ossetia, in the single worst attack in months.

Moscow's forces have been struggling to contain the insurgency in the patchwork of mainly Muslim regions which are among the poorest in Russia. Chechnya itself is now firmly under the control of the Kremlin and its local allies.

Analysts say the regional insurgency is driven by a mix of clan rivalry, frustration at widespread poverty and militant Islamist groups who want to overthrow Moscow's rule. (Reporting by Conor Sweeney; Editing by Giles Elgood)

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