The Russian authorities have ruled out using force to end the siege at a school in North Ossetia.
Families are facing an agonising wait for news
More than 350 pupils and adults are now said to be held hostage - though relatives say the school in Beslan had about 1,000 pupils in total.
Hundreds of parents spent an agonising night outside the school, and correspondents say they are getting increasingly angry at the lack of news.
President Putin said the safety of the hostages was paramount.
"Our main task is to save the life and health of those who have ended up as hostages," he said, having cancelled a trip to Turkey to deal with the crisis.
Some of the hostages have reportedly been able to phone home, describing conditions in the school as tolerable.
The head of the Russian security service in North Ossetia, said there was "no question" of opting for force at the moment.
"There will be a lengthy and tense process of negotiation,"
Valery Andreyev was quoted as saying by Reuters news agency.
North Ossetia's Interior Minister Kazbek Dzantiyev said 12 civilians have been killed since the siege began on Wednesday morning, Russia's Itar-Tass news agency reported.
The hostage-takers began talking overnight to prominent paediatrician Lev Roshal, who helped negotiate the release of children during the siege of a Moscow theatre in 2002.
Mr Roshal said they refused offers to deliver food and water, but he was assured the children were fine. Telephone contact was severed at 0300 (2300 GMT).
The BBC's Damian Grammaticas in Beslan said parents, red-eyed and anxious, spent most of the night around a military cordon set up some 200 metres from the school.
As the wait continued with no news, tempers began to fray, he added. The parents appealed to President Putin to ensure the siege ended without bloodshed.
"We cannot go home and we will not leave. We will wait for an answer," a relative called Zalina told the Associated Press.
"We expect assistance from the authorities... We think that they have to help us. We cannot go home and leave our children there. They are innocent."
There was sporadic gunfire throughout the night.
Masked men and women, wearing bomb belts, burst into the school, whose pupils are aged seven to 18, at around 0930 local time (0530 GMT) on Wednesday.
It was the start of a new term, a day of celebration in Russia traditionally attended by parents.
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 pupils, teachers and parents were herded into the gym. Up to 50 children were reported to have escaped in the confusion.
Trip wires are believed to have been laid around the school, with the attackers threatening to blow it up if stormed by police.
Mr Dzantiyev was quoted as saying on Wednesday they threatened to kill 50 children for every fighter killed.
The hostage-takers are reported to have demanded the withdrawal of Russian troops from Chechnya and the release of Chechen rebels held in Ingushetia.
The Russian authorities are blaming what they call international terrorism, with suspicion falling on Chechen separatists, who have been fighting for independence from Moscow for the past decade.
Chechen rebel leader Aslan Maskhadov has denied that his forces are involved in the siege.
The UN Security Council - at a meeting on Wednesday - condemned the hostage takers "in the strongest terms".
The 15-member council demanded "the immediate and unconditional release of all hostages of the terrorist attack", at a session requested by Moscow.
The US called the siege "callous", and President George W Bush offered "support in any form", according to the Kremlin.
The school crisis came a day after a suspected suicide bombing in Moscow killed 10 people. Last week the mid-air explosions of two passenger planes left 89 dead.
1 - Main entrance
2 - Area where gunfire began
3 - Hostages are being held in the school gym, which has reportedly been packed with explosives and mines
4 - Side entrance
5 - There have been reports of children being used as human shields at the back windows
6 - As the attack began a number of hostages hid in the boiler room and later escaped