The north was considered one of the safer parts of Chechnya
A suicide bomb attack on local government buildings in Chechnya has killed at least 37 people and injured 197 with others missing under the rubble, local officials say.
A lorry loaded with explosives was driven at a compound containing the headquarters of the Federal Security Service (FSB) in the northern town of Znamenskoye, sources in the pro-Moscow administration said.
The blast left a crater at least 15 metres (yards) wide and four deep, gutting buildings at the scene and causing other serious damage within a radius of up to 500 metres.
Russian President Vladimir Putin insisted the incident would not derail peace efforts in Chechnya where rebels are waging a war for independence.
I am looking
for my wife's sister and two daughters here... I am digging
Regional officials said at least one suicide attacker was inside the lorry - said by some sources to have been a military Ural - when it ploughed through a security barrier at 1000 local time (0600 GMT) on Monday.
FSB chief Nikolai Patrushev said the barrier stopped the lorry from reaching the main building and it blew up 30 metres (yards) from the compound's security wall.
But up to 10 buildings inside were completely destroyed or badly damaged, including the FSB and district administration offices and a number of residential blocks.
Most of the dead are said to be civilian staff or local people present on official business, although 10 FSB personnel are also believed to have died.
"I am looking
for my wife's sister and two daughters here," one man told a Russian TV reporter at the scene.
"I am digging
and digging. They are here in this house, I am looking for
them. I will look under the concrete slabs, I don't know
where they can be, but I must look until the end."
The reporter said groans and
tapping on pipes could be heard from under the rubble.
Chechnya's Prosecutor, Vladimir Kravchenko, pointed out that many local civilians would have been visiting the administrative offices on Monday as it was the first working day after Russia's May holiday period.
Dozens of people were rushed to hospitals locally or in Russia's neighbouring North Ossetia region, with 57 in a critical condition.
Monday's attack is a serious blow to Moscow's push for a political solution aimed at ending the war, BBC Moscow correspondent Sarah Rainsford says.
Local government chief Sultan Ahmetkhanov said soldiers at the gates had opened fire on the lorry in an attempt to stop it, but the vehicle tore through the barriers.
Major rebel attacks
Dec 2002 - Suicide attack on Grozny government kills 80
Oct 2002 - Rebels seize Moscow theatre - 120 die in rescue bid
Aug 2002 - giant Russian military helicopter shot down, with at least 118 personnel killed
If it is confirmed that the bombing was perpetrated by Chechen separatists, it would be their deadliest attack since a December suicide bombing in Grozny.
It would also be the most serious attack since a March
referendum tied Chechnya firmly within the Russian federation, while giving the republic some autonomy.
The vote was seen as a victory for Moscow but its validity has been queried by human rights organisations.
"The act... was directed at stopping the process of bringing about a political settlement to the situation in Chechnya," Mr Putin told a meeting of top government officials.
"We cannot allow anything like this to happen, nor will we."
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Russia can decrease the nationalist influence by alienating the rebels from their recruiting base
Correspondents say that northern Chechnya, where the bombing took place, is considered the most stable part of the region.
It was the first area to come under the control
of Russian forces that entered the republic in 1999,
starting the second war in a decade.
Historically, the region was home to a large population of Russians - the Terek Cossacks.