Twenty-five people were killed in two attacks in Afghanistan Saturday, including one in the capital, with the blasts potentially jeopardising attempts by Kabul to persuade the Taliban to join peace talks set for next month.

Witnesses and officials described how the suicide bomber detonated near the Defence Ministry in the centre of Kabul just as offices closed for the day, in an attack later claimed by the Taliban.

"Twelve people, including two Afghan soldiers were killed and eight others injured," a ministry statement said, while a previous toll given by Kabul police chief Abdul Rahman Rahimi stated nine were dead and 13 wounded.

The bomber was on foot, ministry spokesman Dawlat Waziri added.

"I saw wounded civilians and army soldiers. They were begging for help but security forces did not allow common people to help them," witness Sardar Mohammad told AFP.

"The causalities, mostly, were civilians," said another man, Saleh Mohammad. "It was the time when all the people were going home."

Ambulances converged at the site of the explosion as police and the army set up a security cordon.

Analysts have previously observed the Taliban stepping up attacks in the heart of the capital to gain leverage ahead of attempted peace negotiations with the Western-backed government in Kabul, against whom they have been fighting a bloody insurgency for more than 14 years.

Earlier on Saturday a suicide bomber on a motorbike struck at a market in Asadabad, the capital of restive Kunar province, killing 13 people and wounding at least 39.

No group has yet claimed responsibility for that attack, which a spokesman for the provincial governor and a police official both said targeted a tribal leader fiercely opposed to the insurgents, Haji Khan Jan.

The Taliban do not generally claim attacks with high numbers of civilian casualties, saying they only target Afghan soldiers -- "stooges" of foreign powers -- and NATO troops, considered "invaders", as well as symbols of the central government.

But civilians are paying a heavy price in the violence tearing the country apart. The number killed or wounded in 2015 was the highest recorded since 2009. According to a UN report published earlier this month, there were more than 11,000 civilian casualties in 2015, including 3,545 deaths.

- 'Unrealistic' -

The blasts come amid fresh efforts by Afghanistan, China, Pakistan and the US to restart talks aimed at ending the Taliban's long and bloody insurgency in Afghanistan.

Delegates from Afghanistan, China, the US and Pakistan met in Kabul last week for a fourth round of talks aimed at forming a path back to the nascent peace process.

The four countries have called for a direct dialogue between the Taliban and Kabul by next week, but analysts have termed the deadline "completely unrealistic", especially as the insurgents have said they have not been contacted by the quartet.

Kabul has repeatedly called for all Taliban groups to sit at the negotiating table though President Ashraf Ghani has said his government will not make peace with those who kill civilians.

A first round of direct talks was held last summer in Pakistan, but a second edition had been indefinitely postponed by the announcement of the death of Mullah Omar, the Taliban's founder.

His successor, Mullah Akhtar Mansour, is a divisive figure blamed by many insurgents for keeping Omar's death secret for two years. A splinter group formed in December and has challenged Mansour's rule. He was also injured in a firefight among cadres in Pakistan that same month.

Despite the setbacks, the Taliban are far from surrendering. Since the end of the NATO combat mission in Afghanistan in late 2014, they have instead multiplied attacks and offensives on the ground.