But in the new video he said: "I declare that my health is good to serve Allah.

"And I will serve the word of Allah and work to kill the enemies of Allah in all the time that he gives me to live on this earth".

Kavkazcentre.com added that Umarov had described the earlier video, first released on Sunday, as a "fabrication", but offered no explanation on the two differing statements.

Separatist rebellion

Umarov describes himself as the emir of the Caucasus Emirate - a separatist group fighting for independence in Russia's predominantly Muslim North Caucasus, to create a state governed by sharia (Islamic law).

in depth

  Timeline: Attacks in Russia
  The North Caucasus: A history of violence
  Chechnya's battle for independence
  Analysis: Moscow metro explosions

Also known as Abu Usman, Umarov has for years been at the centre of the rebellion in the Caucasus, which has claimed scores of lives annually.

He joined Chechnya's separatists in 1992, fighting in both Chechen wars against Russian forces, and also served as security minister in the province's separatists government from 1996 to 1999.

Umarov claimed responsibility for twin suicide bombings that killed 40 people on Moscow's metro in March and the derailment of a Moscow-St Petersburg train that left 26 people dead in November last year.

He is believed to have been wounded several times in recent years, causing some speculation over whether the rebel leader is still alive.

Russian authorities have prematurely announced his death on a number of occasions.

His separatist movement has evolved from one seeking independence from Moscow to a broader Islamic movement looking to establish an Emirate across the Caucasus mountains.

Last month, the Chechen rebels attacked a hydroelectric plant in the North Caucasus republic of Kabardino-Balkaria, signalling a change in their tactics and making good on a pledge made long ago to attack Russia's economic interests.

The United States has listed Umarov, who is also Russia's most-wanted man, as a terrorist.