Nine Russian soldiers were killed and three were injured when a reinforced concrete wall collapsed on them during a clash with rebels in Chechnya (search), a military spokesman said Tuesday.

Maj. Gen. Ilya Shabalkin, spokesman for the federal forces in Chechnya, said it was unclear what had caused the collapse Monday night. Military investigators were considering several scenarios, including that the wall was brought down by a grenade or another blast set off by rebels.

A number of rebels were killed in the clash, including an Arab mercenary, Shabalkin said.

The soldiers were spending the night in a building on a former poultry farm in the village of Prigorodnoye, four miles south of Grozny (search), which was being used as a temporary barracks. The ITAR-Tass news agency said Chechen police considered the wall collapse an accident.

Maj. Gen. Sergei Surovikin, commander of the 42nd motorized infantry division, which is permanently deployed in Chechnya, said the wall fell because of a direct hit from a grenade-launcher. He said about nine soldiers were killed but the exact number was not clear.

Five to six rebels were involved in the attack, and two of them were killed and one severely wounded, Surovikin said. Soldiers were searching for the rest in a forest outside Prigorodnoye, which is located about a half-mile from the Russian military headquarters at Khankala.

"I declare that for every soldier who was killed, three rebels will be killed," Surovikin said in Grozny.

A cease-fire announced by Chechen rebel leader Aslan Maskhadov (search) was due to expire Tuesday, the eve of the 61st anniversary of the Stalin-era deportation of the Chechens en masse to the barren steppes of Central Asia. The day is also marked as the armed forces' holiday.

Chechnya's Kremlin-backed president, Alu Alkhanov, said Monday that authorities were investigating whether federal forces opened fire without provocation on a car in Chechnya, killing five people, including two presidential security officers.

Alkhanov said on Chechen State Television that the gunfire apparently came from an armored personnel carrier assigned to a checkpoint between the village of Kurchaloi and Geldagan. The attack happened near the checkpoint Friday, he said.

"It does not matter to the population of the republic of Chechnya who has perpetrated this crime — guerrillas or people wearing epaulettes," Alkhanov said in televised remarks. "Crimes committed by the latter deliver a heavier blow to the Chechen people because they hope for their protection."

The Russian military's heavy-handed approach to Chechnya has alienated much of the local population and stoked tension between the military and Chechen officials.

Rights groups say killings of Chechen civilians by Russian soldiers are common, and they have called for more prosecutions. The Russian military insists it is prosecuting abuses.

Alkhanov told Chechen television that the gunmen did not ask the passengers to produce any identification or make any other demands, although it was not immediately clear how he would know that.

Video on Russia's NTV showed bloodied bodies lying in brush near the roadside.