The Afghan Taliban on Saturday said the killing of a brother of their leader in the Quetta mosque bomb attack on Friday would not derail talks with the United States.

Taliban leader Haibatullah Akhundzada was not in the mosque near Quetta on Friday when a bomb went off but his younger brother, Hafiz Ahmadullah, was leading Friday prayers and was killed, several Taliban officials have said.

The bomb blast inside the mosque just before Friday prayers killed four people, including the prayer leader, and left 25 others injured in the Kuchlak area on the outskirts of the provincial capital.

The explosion was carried out with the help of a time device, which was planted under the wooden chair of the prayer leader, when he started delivering the Friday sermon, according to police and bomb disposal squad officials.

There was no claim of responsibility for the blast that came after both Taliban and US officials reported progress in talks on an agreement centred on a US troop withdrawal from Afghanistan in exchange for a Taliban security guarantee.

“If someone thinks martyring our leaders would stop us from our goal they're living in a fool's paradise,” a Taliban leader said by telephone from an undisclosed location.

“We are close to our goals," he said, referring to the talks with the US. He declined to be identified.

The militants have been fighting to expel foreign forces and set up an Islamic state since they were ousted in October 2001, weeks after the September 11 attacks on the US.

Both US negotiators and the Taliban have reported progress after eight rounds of talks since late last year.

US President Donald Trump has made no secret of his desire for a US pullout from Afghanistan and an end to America's longest war. Top US national security advisers briefed Trump on Friday on the negotiations.

Under the expected deal, the Taliban, in exchange for a US commitment on a withdrawal, would guarantee Afghanistan would not be a sanctuary for militants to expand and plot new attacks, both sides have said.

The Taliban are also expected to make a commitment to open power-sharing talks with the US-backed government and agree to a ceasefire.

Presidential spokesman Sediq Seddiqi told a news briefing any deal had to include a ceasefire and a US withdrawal would be based on conditions being met.

'Final round'

A second Taliban official, speaking from Doha, where the Taliban have a political office and where the talks have been held, said preparations were being made for a meaningful final round.

“We've solved most issues, just a few remaining,” said the official, who also declined to be identified.

No date has been set for more talks.

The Taliban were considering a ceasefire in places from which US forces withdraw, the second official said.

Some 14,000 US troops remain in Afghanistan, training and advising Afghan security forces and conducting counterinsurgency operations.

The war has ground into a stalemate with casualties rising among civilians as well as combatants.

White House signals progress

Earlier on Friday, the White House also signalled progress in preparations for a peace deal with the Taliban in Afghanistan, saying that discussions between President Donald Trump and top advisers went “very well”.

Trump had met with national security advisers to discuss the matter. “Discussions centred around our ongoing negotiations and eventual peace and reconciliation agreement with the Taliban and the government of Afghanistan. The meeting went very well, and negotiations are proceeding,” deputy press secretary Hogan Gidley said in a statement.

Trump added on Twitter: “Just completed a very good meeting on Afghanistan. Many on the opposite side of this 19 year war, and us, are looking to make a deal — if possible!”

Washington is keen to end its involvement in Afghanistan, where it has spent more than $1 trillion and Trump has said since the start of his presidency that he wants troops out.