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At least three Russian soldiers have been killed and other people injured in a blast in Chechnya's capital, Grozny.
Local officials said it was a suicide bombing, launched when soldiers left their vehicle.
Differing reports said three or four soldiers were killed, and that there were either one or two suicide attackers.
Earlier, Russian officials said the blast was an accident during a mine-clearing operation.
Russia fought two wars in Chechnya more than a decade ago.
The unrest has since spread to the neighbouring Russian republics of Dagestan and Ingushetia.
"Measures are being taken to identify terrorists and find their possible accomplices," Chechnya's interior ministry said.
"The situation in the city remains calm."
The Russian authorities have been fighting insurgents in the North Caucasus republic since the collapse of the Soviet Union in 1991.
Chechen separatists and Russian forces waged two wars in the 1990s.
Since the second of those wars ended in 2000, the insurgents have increasingly been Islamists and the unrest has spread to other Russian republics in the region, especially Dagestan