The blast left a crater at least eight metres wide

Eyewitnesses say a lorry packed with explosives crashed through entrance gates at the hospital, and a suicide bomber at the wheel blew himself up.

The blast destroyed the four-storey building at a military base in the town of Mozdok, 10 kilometres (six miles) away from the breakaway Russian province of Chechnya.

"We were the first to arrive. Near the checkpoint of the hospital there were charred corpses," a medical worker told the Associated Press news agency.

Medical and rescue teams worked to pull survivors from beneath the rubble, where they could hear people crying out.

Eyewitnesses said the lorry drove towards the military compound at high speed.

"The driver was a middle-aged man. We did not see anyone else inside," one man told the Associated Press.

Judging by the scale of the destruction and the number of people who were in the hospital... the number of casualties will probably be much higher Russian deputy general prosecutor Sergei Fridinsky

However, other reports said the lorry was already parked in the hospital grounds, and possibly set off by remote control.

The blast left a crater at least eight metres wide (26 feet) and three metres (10 feet) deep.

About 150 people - a mixture of soldiers and civilians - were believed to have been in the hospital at the time of the blast.

"Judging by the scale of the destruction and the number of people who were in the hospital... the number of casualties will probably be much higher," Russian deputy general prosecutor Sergei Fridinsky said at the site of the attack.

Health authorities in North Ossetia region have made an urgent appeal for people to donate blood.

And Russian media said a cargo plane carrying medical staff and supplies had been dispatched to the area, along with dogs trained to search for survivors trapped in rubble.

Russian President Vladimir Putin has sent Defence Minister Sergei Ivanov to the scene of the latest attack.

Security breached

The region's Emergency Situations Minister, Boris Dzgoyev, said the hospital building collapsed like "a house of cards" and local police said the building was totally destroyed.

There has been no immediate claim of responsibility for the blast

A woman who lives four kilometres (2.5 miles) from the hospital told Ekho Moskvy radio that windows broke and plaster fell from the walls in her home.

"I saw a big column of smoke," said the woman, identified as Valentina.

The BBC's Sarah Rainsford says questions are been asked about how this could attack could have happened in one of Russia's most heavily-fortified towns.

Mozdok is the headquarters for Russian troops fighting separatists in Chechnya for most of the past decade.

A Russian official told Interfax news agency that the hospital treated Russian servicemen injured during the country's conflict with the Chechen separatists and this may have been why the building was a target.

In June, a suicide bomber from Chechnya blew herself up on a bus in Mozdok, killing 18 people, most of them Russian military police.

There was no immediate claim of responsibility for the attack.

However, Salambek Maigov, Moscow spokesman for moderate rebel Chechen leader Aslan Maskhadov, said the separatist leadership was not involved.

"The Chechen presidency is not responsible for terrorist acts and denounces such acts," he told the French news agency, AFP.

President Putin has scheduled a presidential election in Chechnya for 5 October, but Chechen rebels have rejected the plan and have vowed to resist Russian forces.