Amid the ruins of the bombed-out hospital surrounded by gutted buildings, officials said the attack has left at least 44 dead.

But as rescue workers frantically rummaged through the rubble, there were increasing fears of the toll rising since many more are still believed buried.

Witnesses said an explosives-packed truck driven by a single man smashed through the hospital gates in the town of Mozdok late on Friday, before exploding and bringing most of the building on top of itself.

Most of the dead were Russian soldiers or medical personnel.

Mozdok in North Ossetia, which borders Chechnya, is the site of one of Russia's most important military bases in the Caucasus, and President Vladimir Putin demanded an explanation of how rebels managed to enter it.

"We have to admit... criminals are continuing to exploit weak points in the defences of military bases."

--Deputy General Prosecutor Sergei Fridinsky

Several Russian newspapers suggested the bomber may have bribed guards at the many checkpoints into the city.

Authorities meanwhile detained the head of the hospital in southern Russia, accusing him of "criminal negligence" in the Friday evening strike on the facility, which had treated Russian soldiers hurt in the guerrilla war with Chechen separatist rebels.

Deadly attack

It was the bloodiest Chechen attack since May and the deadliest outside Chechnya itself since October, when rebels took a Moscow theatre hostage and 129 people died when security forces gassed and stormed the building.

Deputy General Prosecutor Sergei Fridinsky said the hospital treated many soldiers injured while fighting separatist rebels in Chechnya.

"This was a well-prepared and well-planned terrorist act," he told NTV television.

"We have to admit... criminals are continuing to exploit weak points in the defences of military bases."

Splinter groups

Salambek Maigov, representative in Moscow of separatist Chechen president Aslan Maskhadov, told Reuters Maskhadov had nothing to do with the blast. But he said he could not speak for other groups from the fragmented Chechen guerrilla forces.

"It is hard to say who is behind this act... but it is not a commander from the official armed forces of Ichkeria (separatist Chechnya). We have never carried out, and do not carry out such acts," he said.

A Russian soldier lies wounded after the explosion. 

Maskhadov was elected in 1997 during a period of de facto independence for Chechnya, but chased from power when Russia launched a new crackdown against separatists in 1999.

Aid

Russian media said a cargo plane carrying medics and supplies had been dispatched to Mozdok within hours, and authorities were appealing for blood donors to come forward.

Defence Minister Sergei Ivanov went to Mozdok and the Kremlin said Putin had sent condolences to victims' families.

Doctors were treating dozens of wounded at remaining hospitals in the area. "We've got 76 (injured) people from the hospital, mainly from the military and the hospital staff," head doctor Vladimir Selivanov told Russian television.

Chechnya is gearing up for a 5 October election to choose a president - a key event in the Kremlin's plans to restore peace but rebels reject the plan and vow to fight on.

Two female bombers blew themselves up last month at an open-air rock festival in Moscow, killing 15 spectators - the first time the Chechen war had come to the capital since the October theatre siege.