The blast that ripped through a nine-story apartment building here early today was caused by a bomb with the equivalent of more than 440 pounds of explosives planted in a first-floor store, Russian authorities said.

The blast in southeast Moscow killed 40 people and injured 152 others, but dozens more were believed to be buried under the rubble. The midnight explosion threw debris more than 300 feet from the building, and fire and thick smoke hampered rescue work.

An anonymous caller told the Russian news agency Interfax the blast was in retaliation for Russia's recent bombing of villages in Chechnya during the battle with Chechen rebels who have crossed into the southern region of Dagestan. There was no other claim of responsibility.

"Visual signs suggest that it was a terrorist act similar to the one carried out in Buinaksk," Moscow Mayor Yuri Luzhkov said, referring to a car bomb that wrecked a military barracks in Dagestan Saturday.

Initial reports attributed today's blast to a natural gas leak. But by nightfall, authorities said they had established that it was a bomb and said they had identified suspects.

"The building has gas, so one of the causes could have been gas. But in the first hours of work, it became clear that it did not look like a gas explosion," said Sergei Shoigu, minister for emergency situations. "The explosion was too powerful and the damage too big."

Luzhkov told reporters the bomb's substance was hexogen, an explosive used only in bombs and shells.

Vladimir Minayev, head of the main investigative department of the Russian prosecutor general's office, pointed to the effects of the blast. "Look at the distance where parts of the building were thrown," he said.

The blast crumpled two stairwells and the connecting apartments on each side of them at 19 Guryanov St. The two wings of the apartment building, made of precast concrete slabs, were left standing. Some of the casualties were in a neighboring building that was showered with debris.

Additional security measures were imposed in Moscow, and police were ordered to full alert with 12-hour shifts and all leaves canceled.

Meanwhile, Russia continued to suffer losses in the combat in Dagestan. An Su-25 plane crashed after its pilot ejected, but officials said the plane was not shot down. Russian forces said they seized several strategic hilltops, but the toll was high--10 killed and 50 wounded. Since the battles with Chechen rebels began in August, Russia has lost 149 soldiers and 522 wounded. The rebels are seeking an Islamic state.