MOSCOW, Aug 21 (AFP) - A powerful blast which officials said was caused by a methane gas leak ripped through a Moscow apartment building late Tuesday, killing at least six people and reviving memories of a series of deadly apartment block bombings that terrorized the Russian capital three years ago.

Officials said 12 people were injured in the blast, with three of them taken to hospital in a serious condition.

One woman received burns over 80 percent of her body, Russian television reported.

The toll could rise as rescuers aided by sniffer dogs searched for bodies beneath the rubble, officials said.

Moscow police chief Vladimir Pronin told the ITAR-TASS news agency that experts believed "a poowerful gas explosion, probably on the ground floor" caused the blast.

Russian intelligence services (FSB) said the blast was "probably due to a gas explosion and not an attack."

Immediately following the blast fears were raised that it could herald a resumption of the series of deadly apartment block explosions which killed some 300 people in Moscow and southern Russia almost exactly three years ago.

The Russian authorities used the blasts as justification for a military intervention in the separatist republic of Chechnya, launched on October 1, 1999 and still in progress.

However Moscow deputy mayor Valery Shantsev stressed that no traces of explosives had been detected despite witnesses earlier saying they had smelt gunpowder in the air.

Forty-eight people lived in the apartment block, Moscow town hall spokesman Sergei Tsoi said.

Of these, 22 residents were evacuated unhurt and 12 others were out of the city at the time, ITAR-TASS quoted him as saying.

Senior officials flocked to the site of the blast where dozens of shocked people, most of them evacuated from nearby houses, stood watching helplessly as emergency workers continued their frantic search through the debris.

According to Russia's Emergency Minister Sergei Shoigu, who rushed to the scene to coordinate rescue efforts, up to 15 people could still be buried under the rubble, the Interfax news agency reported.

The explosion caused all five floors of the brick building on Akademika Korolyova street near Moscow's Ostankino television tower to collapse.

Media speculation that the blast could have been a terrorist act were heightened by a report that it occurred in an apartment rented by a family of Caucasian origin who left the building earlier in the day.

But Shoigu, the FSB security services and Moscow police insisted that the blast was caused by a methane explosion.

No trace of explosives have been found in the ruined apartment block, emergency officials said.

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