The Taliban on Tuesday denounced as a sham a presidential election due next month, saying their fighters would do everything they could to block it, while urging people to stay away from rallies that could be attacked.

The warning came as both the United States and the Taliban reported progress in talks on a deal that is expected to bring a US commitment to withdraw its troops in exchange for a Taliban promise the country will not be used to plot terrorist attacks.

The Taliban called for a boycott of the election scheduled for Sept 28, and said foreign powers should instead focus on the effort to negotiate the deal with the US on “ending the occupation of Afghanistan”.

“This election process is nothing more than a ploy to deceive the common people ... for satisfying the ego of a limited number of sham politicians,” the Taliban said in a statement.

President Ashraf Ghani is widely expected to win a second term and has insisted the vote must go ahead as scheduled.

The US-trained former World Bank official came to power in 2014 after winning a bitterly disputed election marred by accusations of cheating.

The Taliban said their fighters would block the vote and warned of attacks. “To prevent losses ... from being incurred by our fellow compatriots, they must stay away from gatherings and rallies that could become potential targets,” the militants said.

Ghani's office, responding to the threat, said the people had the right to choose their leader and the government was ready to hold a transparent election across the country.

“They should show peace through their actions and not threaten people,” the office said. Ghani and his government are not involved in the peace talks because the Taliban refuse to deal with a government they say is a US puppet.

Ghani's running mate, who last week survived a militant suicide attack on his Kabul office, condemned the Taliban as “sham Afghans”.

Progress on pact

The rights group Amnesty international also condemned the Taliban threat. “At a time when the Taliban claims to be pursuing peace, it is threatening to carry out war crimes by attacking civilians at election rallies,” the group's South Asia researcher Zaman Sultani said in a statement.

Both the US peace envoy leading talks with the Taliban, and the Taliban, reported progress in their talks in Qatar.

The US envoy, Zalmay Khalilzad, said on Twitter late on Monday, excellent progress was being made while agreement on details was essential. He did not elaborate.

The Taliban said they were expecting a positive outcome within days.

“Extraordinary progress has been made,” said Suhail Shaheen, the spokesman for the Taliban political office in Qatar.

Shaheen said “a complete Islamic system” had been the goal over 40 years of war and the question of elections would be discussed in so-called intra-Afghan dialogue, which is meant to take place after a deal is struck with the US.

Despite the talks, there has been no let-up in violence. Two people were killed and seven were wounded in a bomb attack on a government vehicle in Kabul on Tuesday, authorities said.

Four people were killed and 25 wounded in a bomb blast in the western city of Herat on Monday, provincial authorities said. The militant Islamic State (IS) militants claimed responsibility on their Amaq news agency.