Ashraf Ghani is hoping to fend off challenges from 17 other candidates to score a second term at twice-postponed presidential elections now slated for September 28.

"Peace is coming, and the negotiations will take place," Ghani said at a rally marking the start of two months of campaigning.

His comments came a day after his peace minister, Abdul Salam Rahimi, said direct talks would take place with the Taliban within two weeks as part of a larger, US-led push for peace.

Such a development could be crucial, as the Taliban -- who now control or influence about half of Afghanistan -- have so far refused to speak to Ghani's government, as they consider the Kabul administration illegitimate.

War aside, the country faces a host of major issues ahead of the election, including rocketing crime, a lacklustre economy, soaring unemployment, and crumbling infrastructure.

Voters are despondent about the prospects of a fair election, and many worry about a repeat of the sort of violent attacks on previous polls by the Taliban and other insurgent groups trying to undermine Afghanistan's fragile democracy.

Ghani insisted this year's election would be "clean", but distrust is rife.

Sayed Jan, a 27-year-old student, said he won't be voting as he has lost faith since the 2014 election that was mired in allegations of massive fraud and ballot stuffing.

"We have been betrayed by the candidates in the past, we can not trust them this time," he told AFP.

"We need peace in Afghanistan instead of elections. Even if I vote, the election will be fraudulent."

- 'Opportunities for peace' -

In Kabul, security forces fanned out across the city as leading candidates held rallies. Campaign posters of Ghani, his main rival Abdullah Abdullah, and other candidates were unveiled across the country.

Abdullah is currently serving as the president's chief executive under an awkward power-sharing arrangement brokered by the US after the 2014 election.

"It is our national and religious duty to take advantage of any opportunities for peace," Abdullah told a campaign rally.

One crucial issue is that the elections happen at all: they were postponed twice this year and further delays could lead to more distrust.

Despite Ghani's claim that a summit between his government and the Taliban would take place shortly, the insurgents said they would only talk to Kabul after the US had announced a timeline for a withdrawal of foreign forces -- a key part of any deal.

"The Kabul administration will be considered a political side, just like others, and not a government," Taliban political spokesman Suhail Shaheen wrote in Pashto on Twitter.

Diplomatic sources have told AFP the Afghan-Taliban talks are scheduled to begin in Oslo on August 7.

US peace envoy Zalmay Khalilzad, who is leading a push to forge a peace deal with the Taliban, said Saturday that new "intra-Afghan" negotiations would only take place after the US and Taliban had concluded their own agreements.

He is expected to fly to Doha next week for what would be the eighth round of direct US-Taliban talks.

Other presidential candidates include Ghani's former national security advisor Hanif Atmar and former warlord Gulbuddin Hekmatyar, nicknamed the "Butcher of Kabul" for his alleged role in the killing of thousands of people in the capital in the 1990s.