Friday, August 18, 2006 Posted: 1231 GMT (2031 HKT)
A government spokesman initially said it was Doku, the most powerful among surviving rebel field commanders and the head of resistance movement from June, who had surrendered. But he later retracted his statement.
"Initially we had information that it was Doku Umarov, but later it turned out that it was his younger brother," the spokesman said by telephone.
He did not give the name of Umarov's brother or any other details.
Russia is striving to end separatist resistance in Chechnya, where it fought two bloody wars since 1994. It issued a call for rebels to disarm voluntarily in July, after two of the most prominent separatist leaders were killed.
In March, Russian troops killed rebel leader Aslan Maskhadov.
In July a prominent warlord Shamil Basayev, blamed by Russia for carrying out attacks against civilian targets outside Chechnya, died in a truck blast, described by Moscow as a successful operation by its security forces.
Basayev had claimed responsibility for a bloody school siege in the southern town of Beslan in 2004, in which more than 300 people died, half of them children.
Doku Umarov remains the only separatist field commander who has authority among rebels and is capable of uniting their forces.