Rescue workers are sifting through debris in an attempt to find survivors of a bomb attack on a military hospital in southern Russia that has left about 30 people dead.
The blast left a crater at least eight metres wide
Up to 650 soldiers and rescue workers are searching through the wreckage of the four-storey building at a military base in the town of Mozdok, 10 kilometres (six miles) away from the breakaway Russian republic of Chechnya.
The building was destroyed after a lorry packed with explosives crashed through the entrance gates and a suicide bomber at the wheel blew himself up, Russian media said.
Experts told the Russian news agency Interfax that there may still be people alive underneath the debris.
Rescue workers are pausing for five-minutes every hour to listen for sounds of life from the rubble, the agency reports.
Although Russian media have reported that the attack was the work of a suicide bomber, 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 - 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.
The region's Emergency Situations Minister, Boris Dzgoyev, said the hospital building collapsed like "a house of cards".
The blast was heard up to nine miles (15 km) away.
One woman told the BBC that the mushroom cloud caused by the blast was up to 650 feet (200m) high.
There has been no immediate claim of responsibility for the blast
"The blast was very violent, everything was shaking within the distance of six kilometres," she said.
"I saw four ambulance cars, they were carrying injured people."
The BBC's Sarah Rainsford says questions are being asked about how this 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.
Mr Fridinsky said 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 rebels have rejected the plan and have vowed to resist Russian forces.