War-torn Afghanistan has delayed its presidential election until September 28, officials announced on Wednesday, the second time the ballot has been put back and five months later than it was originally scheduled to be held.

The announcement by the Independent Election Commission (IEC) comes after speculation that the vote would be postponed to create space for US-led efforts to end the 17-year war with the Afghan Taliban.

The presidential ballot was originally scheduled for April 20, then delayed to July 20.

Many observers had considered both dates unrealistic given the IEC is still finalising results of October's shambolic parliamentary elections.

The IEC said in a statement on Wednesday that the vote had faced "numerous problems and challenges ... therefore holding the elections based on the timelines previously announced is not possible."

"In order to better implement the rule of election law, ensure transparency as well as voter registration, the presidential election, provincial council election as well as the parliamentary election of Ghazni province, will be held on September 28," the statement said.

The election should go ahead, it added, "provided all relevant sides, especially the government and the international community, provide the IEC with the required budget on time".

A spokesman for President Ashraf Ghani, who plans to seek re-election, said the government respects the decision and is "fully prepared to cooperate with the IEC".

The statement did not mention the ongoing talks between the US and the Taliban, which have caused unease in Kabul as Washington seeks a way out of its longest war.

Taliban talks

Afghan officials have voiced frustration at being sidelined in the process, complaining it undermines the legitimacy of the government in Kabul, which the Taliban dismisses as a puppet regime.

Earlier this month a close Ghani aide sparked a diplomatic spat with Washington by criticising the talks.

"We don't know what's going on. We don't have the kind of transparency that we should have," Hamdullah Mohib, Afghanistan's national security advisor, told media during a visit to Washington.

"The last people to find out are us."

US special envoy Zalmay Khalilzad, who is leading the talks for Washington, said after the latest round ended in Doha earlier this month that "real strides" had been made but no agreement was reached on a timetable for a troop withdrawal.

He spoke of an agreement "in draft" on the issues of counter-terrorism, assurances from the Taliban and troop withdrawal, and suggested the next phases would be intra-Afghan dialogue and a ceasefire, but stressed that nothing was finalised.

Many Afghans are worried that Ghani's fragile unity government would collapse if US troops pulled out, enabling the Taliban to return to power and potentially sparking another bloody civil war.

There also are concerns the presidential election, which will now be held nearer the Taliban's traditional fighting season, could unleash a wave of deadly violence as militants seek to disrupt the vote.

On Wednesday — the eve of Nowruz, the Persian new year — the interior ministry said the government would "increase" its offensive operations against the insurgents in the coming 12 months.

Also on Wednesday, the Taliban released 58 soldiers they had captured in Badhgis province, the insurgents and the provincial governor's spokesman said.