The government's press secretary, Lema Gudayev, did not disclose the militant's name, but said he had also been an aide to Umarov. No further details have yet been made available.
The Chechen government earlier reported that Umarov, the most influential figure among militants in Russia's troubled republic since the killing of terrorist No. 1, Shamil Basayev in mid-July, had surrendered.
The report on Umarov's possible surrender came after several militants from his group laid down their arms several days ago.
After the death of Basayev, who claimed responsibility for the Beslan school massacre and other atrocities, the Kremlin declared an amnesty until August 1 for militants active across the North Caucasus to lay down their arms promising them unbiased investigation. The deadline was later extended until September 30.
Chechen Prosecutor Nikolai Kalugin said Friday 119 gunmen had surrendered in Chechnya alone since the amnesty was declared.
"Forty-two militants have been checked through law enforcement databases, and a decision has been made not to open criminal cases against them," Kalugin said, adding that the other gunmen were still undergoing checks and one had been put in custody.
But militants have staged three attacks this month in the neighboring Caucasus republics of Daghestan and Ingushetia, targeting law officers and killing five and wounding 13 people.
A source in the Chechen government earlier said Chechnya's prime minister, Ramzan Kadyrov, had contacted Umarov via his family and other mediators.
Doku Umarov served under warlord Ruslan Gelayev, until he was killed in 2004, and is believed to have been involved in the killings of 30 police officers and servicemen. In 2004, he was in the Basayev-led group that raided Ingushetia, where police said militants attacked 19 police precincts and prisons.
Sporadic fighting in Chechnya - although the active phase of the anti-terrorism campaign has been declared over - has hampered efforts to rebuild housing and infrastructure, create jobs and incite separatists to return to peaceful life in the republic after a decade of warfare.
