* Blast kills four in mosque in Pakistan's Balochistan
* Brother of Taliban leader killed, son wounded
* No claim of responsibility
* U.S., Taliban cite progress in deal on US withdrawal
* (Adds comment from Taliban official, detail)
By Gul Yousafzai and Abdul Qadir Sediqi
QUETTA, Pakistan/KABUL, Aug 16 (Reuters) - The brother of the leader of the Afghan Taliban was among at least four people killed in a bomb blast at a mosque in Pakistan on Friday, two Taliban sources told Reuters, an attack that could affect efforts to end the Afghan war.
There was no immediate claim of responsibility for the blast which took place as the Taliban and the United States are in the final stages of talks on an agreement that would see America withdraw its troops from neighbouring Afghanistan.
Police said more than 20 people were wounded and the death toll could rise. The imam of the mosque, 25 kilometres (15 miles) from the southwestern city of Quetta, was among those killed, police said.
Taliban leader Haibatullah Akhundzada was not in the mosque when the bomb went off but his younger brother, Hafiz Ahmadullah, was among those killed.
“We lost younger brother of Sheikh Haibatullah Akhundzada in a bomb blast," said a senior member of the Taliban who declined to be identified.
Akhundzada's son was wounded, one of the sources said.
The mosque and an adjoining religious school were known to be visited by members of the Afghan Taliban. Akhundzada used to teach and lead prayers at the complex, the senior Taliban member said.
Pakistani police did not confirm the identity of any of the victims.
"It was a timed device planted under the wooden chair of the prayer leader," said Abdul Razzaq Cheema, chief of police in Quetta, capital of Balochistan province.
One of the sources, who visited the site after the blast, said security at the mosque was always very tight.
Separatist insurgents and Islamist militants are active in Balochistan but many in Pakistan are likely to suspect the Afghan government, which is battling the Taliban in Afghanistan, of involvement in the blast.
There was no immediate comment from the Afghan government.
PROSPECTS FOR PEACE
Pakistan has promised to help the United States end the Afghan war, its longest conflict, and both U.S. and Taliban negotiators have recently reported significant progress in their talks in Qatar.
But Friday's blast will raise concerns about prospects for peace.
The Taliban, in exchange for a U.S. troop withdrawal, are expected to guarantee that Afghanistan will not be used for international terrorism.
The militants, fighting to expel foreign forces and set up an Islamic state, are also expected to make a commitment to open power-sharing talks with the U.S.-backed government and begin a ceasefire.
U.S. President Donald Trump is keen to bring the troops home but many Afghans fear a U.S. troop withdrawal would leave the government battling the Taliban alone.
The United States has about 14,000 troops in Afghanistan, with a focus on training Afghan forces and counter-terrorism.
The Taliban roam through more territory now than they have since the United States ousted them from power in 2001 for sheltering al Qaeda militants behind the Sept. 11 attacks, and fighting between government forces and the insurgents has been heavy. (Reporting by Gul Yousafzai in Quetta and Robert Birsel in Kabul Writing by Saad Sayeed Editing by Alasdair Pal, Stephen Powell and Frances Kerry)