A bomb blast killed at least five people and wounded 34 in the western Afghan city of Herat on Sunday, raising fresh concerns about security for elections in October.
The United Nations said the blast was the latest in a string of violent incidents, which underscored the need for immediate deployment of more international troops to protect the October 9 presidential vote and parliamentary elections in April.
The explosion came less than two days after the announcement of the poll dates and just ahead of a ceremony to mark the start of disarmament of Herat's militia forces, part of a drive seen as crucial to creating conditions for a free and fair vote.
The bomb exploded outside a military post near a busy morning market in the ancient city, which is not far from the Iranian border, provincial spokesman Ghulam Mohammad Masoan said.
All victims were civilians and included women and children. Doctors said the toll could rise as some of the wounded were critically hurt.
"This act of terrorism is the work of Afghanistan's enemies, who are desperately trying to derail Afghanistan from the path of reconstruction, peace and democracy," President Hamid Karzai's spokesman said in a statement.
Herat police chief Zia Uddin Mahmodi said the bomb might have been hidden in a bucket, and the Taleban could be to blame.
Lieutenant-Colonel James Hand, head of a US civilian-military team helping with security in Herat, said there were reports it was a bicycle or a motorcycle bomb. He said he did not know who carried out the attack.
DISARMING FIGHTERS: Masoan called the bomb the work of "those people who do not want a stable Heart."
Herat has long been seen as one of the more stable parts of the country, but in March fighting erupted between forces loyal to a government commander and Khan's militia, in which Khan's son, Aviation Minster Mirwais Sadiq, was killed.
Karzai sent national troops to intervene but commanders loyal to Khan, who forced out those of the rival commander, said they were not needed.
The ceremony marking the start of disarmament of Khan's militia began three hours after the blast, at a military base 10 km (six miles) away.
Disarming of irregulars commanded by strongmen such as Khan has moved slowly, with only about 10,000 demobilised out of a total of some 50,000.
Some commanders, including Khan, have warned that disarming their forces will increase instability, especially while a new national army - only 12,000 strong - is so small.
Analysts say some commanders will also have been angered by the decision to hold the parliamentary polls six months later than the presidential vote, seeing an early election as their best chance to consolidate their positions.
At a news briefing, UN Special Representative Jean Arnault it was possible blasts like the one in Herat might be the work of those opposed to disarmament.