A siege at a school in south Russia has continued into the night, with armed attackers holding at least 200 people hostage, many of them children.
Families are facing an agonising wait for news
Several people were said to have died after about 17 men and women, wearing bomb belts, seized the school in North Ossetia on Wednesday morning.
Officials say they are in contact with the gang, Russian media report.
The attackers' demands are said to include the withdrawal of Russia troops from neighbouring Chechnya.
Pupils have reportedly been lined up against the windows to prevent an assault by security services, says the BBC's Moscow correspondent Damian Grammaticas.
The siege comes amid heightened security across the country. A suspected suicide bombing in Moscow on Tuesday night killed 10 people, and last week the mid-air explosions of two passenger planes left 89 dead.
Suspicion for these attacks has fallen on Chechen separatists, who have been fighting for independence from Russia for the past decade.
Chechen rebel leader Aslan Maskhadov has denied that his forces are involved in the siege.
Russian security forces and police troops are surrounding the school in Beslan, a town 15km (10 miles) north of Vladikavkaz, capital of the North Ossetia republic, which borders Chechnya.
Distraught families rushed to the scene, where sporadic gunfire could be heard.
"Every gunshot I hear is like a shot in my heart," one woman whose child was inside the school told Reuters news agency.
The attackers are believed to have laid trip wires, saying they will blow up the school if stormed by police.
The gang has also threatened to kill children if any of them are hurt by security forces, a regional official said.
"They have said that for every fighter wiped out, they will kill 50 children, and for every fighter wounded, 20," North Ossetia's Interior Minister Kazbek Dzantiev was quoted by Reuters as saying.
There is confusion over the exact number of hostages, but Mr Dzantiev said probably between 300 and 400 people were inside.
Wednesday was the beginning of the school term for millions of children across Russia, and parents also attend what is traditionally a day of celebration.
The attackers burst into the school, whose pupils are aged seven to 18, at around 0930 local time (0530 GMT), shortly after a ceremony welcoming in the new school year.
"We were standing in lines next to the gates. Music was being played," pupil Zaurbek Tsumaratov said after managing to escape.
"I saw three people running in there with sub-machine guns. At first I simply thought it was a joke. Then they began to fire in the air and we ran away."
Up to 50 children were able to flee in the confusion, reports say.
Russian President Vladimir Putin broke off a working holiday on the Black Sea to return to Moscow to deal with the crisis.
RECENT DEADLY ATTACKS
31 Aug 2004 - Suicide bomb kills 10 at north Moscow train station
24 Aug 2004 - Two planes crash after leaving same Moscow airport, killing 89
May 2004 - Chechen president killed in blast at stadium in Grozny
Feb 2004 - Bomb attack kills at least 39 people on Moscow underground
Dec 2003 - Female suicide bomber kills five near Red Square
The UN Security Council in New York is meeting at 2100 GMT at Russia's request, to discuss the wave of recent attacks in the country.
It is not yet clear who the hostage-takers are, but correspondents say the siege bears the hallmarks of Chechen rebels.
The rebels have a history of taking hostages to draw international attention to their independence demands - the most notorious incident being the Moscow theatre siege in 2002, which led to the deaths of 129 hostages.