Mohammad Tayab Agha, head of the Taliban's political office, said in a statement today that the movement made a "historic mistake" by concealing the Mullah Omar's death, which it only confirmed last week.
Afghan authorities say the Taliban leader died more than two years ago.
Mullah Akhtar Mansoor, who had served as Mullah Omar's deputy, was chosen to succeed him by the group's Supreme Council, meeting in the Pakistani city of Quetta.