"Baitullah Mehsud is alive but he is seriously ill," Mohammad told the AFP news agency by telephone from an undisclosed location.
"In his absence I announce, as vice-president of the TTP (Tehrik-e-Taliban), the takeover of his leadership.
"Two days ago our shura (committee) held a meeting in which my leadership was endorsed."
Mohammad said his appointment was only temporary until the Taliban council could reach a final decision on who would replace Mehsud.
He told AFP that Hakimullah Mehsud and Waliur Rehman, two other top Taliban commanders, had endorsed his leadership.
The announcement comes days after Pakistani security forces arrested Maulvi Omar, a spokesman for the group.
Omar frequently contacted journalists to issue statements claiming responsibility for suicide bombings and other attacks across the country.
Intelligence officials on Tuesday said Omar confirmed under questioning that Mehsud was dead.
Richard Holbrooke, the US special envoy to Pakistan and Afghanistan, said during a visit to Pakistan at the weekend that Mehsud was "gone" and it looked as if there was a struggle for succession among his commanders.
The Pakistani military has been targeting Taliban fighters in the South Waziristan in recent months in preparation for what military officials have said will be a major offensive to push them out of the region.
However, Nadeem Ahmed, an army commander, said on Tuesday that the military would need months to prepare for an offensive in South Waziristan due to shortages of "the right kind of equipment" and helicopters being used elsewhere in the country.