The blast, coming on the first working day following a series of state holidays, underlined the violence that continues to roil Chechnya even as the Kremlin claims the more than 3½-year-old war is winding down and normal life is returning.
Maj.-Gen. Ruslan Avtayev, the chief of the Chechen branch of the Emergency Situations Ministry, said the blast completely destroyed the two-story building housing the office of the Federal Security Service, the intelligence agency that is leading Russia's campaign in Chechnya, in the town of Znamenskoye. It also damaged four nearby administrative buildings.
At least 30 people were killed, said Avtayev and Sultan Akhmetkhanov, the head of the Nadterechny region, where the blast occurred. More than 100 people were hospitalized, Avtayev said. The two officials said eight residential houses were also damaged.
Other officials gave even higher estimates of the toll.
An official in Chechnya's Justice Ministry, who spoke on condition of anonymity, said at least 50 people were killed. He said the Security Service building, the district police headquarters and the local administration building were in ruins and six apartment houses were badly damaged. The government buildings were full of people who had returned to work following the May holidays, he said.
There was no claim of responsibility, but Chechnya administration chief Akhmad Kadyrov blamed rebels in the region for the attack and suggested one of two separatist leaders, Aslan Maskhadov and Shamil Basayev, may have been behind it, according to Interfax.
Federal Security Service chief Nikolai Patrushev said the blast occurred after a Kamaz truck was halted at a security barrier 30 yards from a concrete wall that protects the government buildings, the Interfax news agency reported.
The blast left a crater up to 53 feet wide and six-feet deep, Akhmetkhanov said, and other officials said it shattered windows 500 yards away. Patrushev said some of the victims were from the security services but most were civilians.
President Vladimir Putin ordered the Prosecutor General's Office and the Federal Security Service to come up with an investigation plan.
"Such actions are intended to stop the process of a political settlement of the situation in Chechnya. We cannot and will not allow anything of the kind," Interfax quoted Putin as saying during a regular meeting with members of the government.
Monday's blast was the latest in a string of truck- and car-bomb explosions in Chechnya, a technique the outmanned and outgunned rebels have used to strike at Russian forces and their Chechen colleagues.
In December, a truck-bomb attack on the headquarters of the Moscow-backed Chechen administration in the capital Grozny killed at least 70 people. The truck had passed through numerous checkpoints and the blast exposed the still-fragile state of security even in the most heavily guarded part of the war-shattered region.
Kadyrov said the Znamenskoye attack showed that Russian and regional Chechen security services are unable to prevent all Chechen rebel attacks.
"It is necessary to be more alert and responsible, so that no cars with explosives can travel on the territory of the republic," Interfax quoted him as saying.
Northern Chechnya 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.
Znamenskoye houses a large refugee camp and has served as headquarters for international human rights monitors.
Russia's government minister for Chechnya said it would be "senseless" to increase the size of the Russian military contingent in the region — tens of thousands of troops — as a result of the attack.
"Large-scale hostilities are long gone, and it is impossible to prevent a terrorist attack with a large number of troops," Interfax quoted Stanislav Ilyasov as saying. He said halting terror attacks would require "targeted operations by the secret services."