The brother of Mullah Mohammad Omar on Sunday joined a growing chorus of opposition to the opaque selection of the late Taliban leader's successor, indicating widening rifts within the militant group as it weighs whether to revive peace talks or intensify its 14-year insurgency in Afghanistan.

As the leadership crisis deepened, the Taliban released a statement from one of its most notorious commanders pledging loyalty to Mullah Akhtar Mohammad Mansoor, who was chosen to lead after the death of the Taliban's reclusive, one-eyed founder was announced last week.

The statement quoted Jalaluddin Haqqani, the head of the Haqqani Network, a Pakistan-based outfit blamed for scores of complex attacks on U.S. and NATO forces in Afghanistan, as calling for unity. The Taliban denied recent rumors that Haqqani, like Mullah Omar, had died in secret. Haqqani's son Sirajuddin was named Mullah Mansoor's deputy after his promotion.

The loss of Mullah Omar has raised concerns of a succession crisis that could splinter the group between relatively moderate figures who back Pakistan-mediated peace talks and more radical field commanders committed to overthrowing the Kabul government and reverting to the harsh Islamic rule of the 1990s.

Those commanders have made steady gains in recent months across northern Afghanistan -- far from the group's traditional heartland -- as Afghan security forces have struggled in the absence of U.S. and NATO combat troops, who switched to a support and training role at the end of last year.

The Taliban announced that Mullah Mansoor was their new leader on Thursday and released a purported audio statement from him on Saturday in which he called for unity and warned of enemy propaganda aimed at dividing the group. The Taliban acknowledged Mullah Omar's death last week, after the Afghan government said he had died in a Pakistani hospital two years ago.

The Taliban deny Mullah Omar ever left Afghanistan, but the secretive nature of his death raised the possibility that the senior Taliban leadership -- a Supreme Council with just seven members -- had concealed his death from the wider movement, which has tens of thousands of fighters.

Mullah Omar's brother on Sunday joined a growing challenge to Mullah Mansoor's leadership, telling The Associated Press he had been "selected" by a small clique of his own supporters.

Mullah Abdul Manan's comments came after Mullah Omar's son, Yacoub, also said the new leader did not have the support of the wider Taliban.

"There should be a (grand council), so everyone has a chance to choose their own leader. I do not accept this selection of Mullah Akhtar Mansoor because only a few chose him," Mullah Abdul Manan said.

If Mullah Mansoor fails to hold the movement together, the ultimate beneficiary could be the Islamic State group, which has established a small but growing presence in Afghanistan over the past year, in part by recruiting disillusioned Taliban fighters.

An internal split could also jeopardize peace talks which began last month but were indefinitely postponed after the announcement of Mullah Omar's death. Mullah Mansoor is widely seen as having pushed the Taliban into the negotiations at Pakistan's bidding.

Afghan President Ashraf Ghani's office said Sunday that he was in touch with Pakistani officials about "bringing peace and prosperity to both counties and to the region."

The statement made no specific mention of the Taliban or Mullah Omar, who is widely thought to have been sheltered by Pakistan after he fled across the border during the 2001 U.S.-led invasion of Afghanistan.

The statement said Ghani spoke to officials by video link from Germany, where he is recovering from foot surgery. He is expected to return to Kabul in the coming days, his office said.

Waheed Muzhda, a former bureaucrat in the Taliban's 1996-2001 administration and now a political analyst, downplayed the importance of Mullah Omar's relatives to the question of succession.

"Decisions made by the Taliban are made according to religious principles, not according to an inheritable legacy, so it won't be considered necessary for Mullah Omar's son to take the leadership role," he said.

"The Taliban is not a political organization, they are fighting jihad against the Afghan government, and so need a person with experience and expertise in many things," he said.