The Mi-24 was downed with a portable missile south of the regional capital Grozny, investigators said on Monday.

Large numbers of such missiles have made their way from old Soviet stockpiles to the fighters in Chechnya in recent years, and have often been used against Russian aircraft.

The worst incident to date is the destruction in 2002 of a troop-laden Mi-26 transport helicopter, resulting in the deaths of 115 people.

Stepping up fight

Meanwhile, President Vladimir Putin pledged on Monday to step up Russia's fight against separatists after the Beslan school siege.

"Terrorists must be annihilated in their dens and, if necessary,
must be attacked abroad"

Vladimir Putin,
Russian president

"Terrorists must be annihilated in their dens and, if necessary,
must be attacked abroad," Putin told a government meeting in Moscow, echoing earlier threats by the military leadership to hit separatists' bases outside the country.

Russia routinely refers to Chechen separatists as terrorists.

Existing anti-terrorist measures "have not achieved required results in liquidating terrorism and its sources", he added.   

The Russian leader also pledged to overhaul state policy towards the explosive North Caucasus region, where many attacks have taken place, and where fighters have prepared devastating attacks in other parts of Russia, including the capital.

In a major change to the Russian electoral system, Putin also has said heads and presidents of the country's 89 regions and republics will in future be appointed in Moscow rather than locally elected.