Image caption The Taliban have vastly strengthened their hand on the battlefield in recent years

The Afghan Taliban spokesman has said one of its founding members has been released from detention in Pakistan.

Mullah Abdul Ghani Baradar served as second-in-command under Mullah Omar and co-ordinated the group's military operations in southern Afghanistan.

He was arrested eight years ago in the Pakistani city of Karachi.

Correspondents say his release could be linked to US efforts to revive peace talks between the militants and the Afghan government.

"He wasn't released because he was ill," a Taliban source told the BBC.

"In fact Pakistan also wants him to play a role in peace talks. He is in good shape and is expected to play a role in the peace process."

Taliban officials secretly met a senior US diplomat in Qatar, it emerged in July. The Islamist group's power and reach have surged since foreign combat troops left Afghanistan in 2014.

Two other senior members of the movement were also released by Pakistan, Tolo news reported.

The move came after recent visits to Kabul by US special representative Zalmay Khalilzad to discuss peace talks.

Media playback is unsupported on your device Media caption Taliban "threaten 70% of Afghanistan", BBC investigation finds

Two unnamed Pakistani intelligence officials also confirmed that Mullah Baradar had been freed "after high-level negotiations", the Associated Press reports.

It said Mullah Baradar's release followed a visit to Pakistan by the foreign minister of Qatar, where the Taliban have a political office.

The US invaded Afghanistan in 2001 in the wake of the 11 September attacks, ousting the Taliban from power and setting the stage for what has turned into a 17-year conflict.

President Donald Trump re-committed the US to the war in a new strategy last year, and stepped up air strikes against the militants to try to force them to the negotiating table.