A police official in western Afghanistan's city of Herat says a large bomb exploded in a crowded district, killing at least five people, including a child, and injuring more than 20. The blast coincides with a ceremony to mark the disarming of a local militia.

Herat Police Chief Ziauddin Mahmoudi says the device had been placed in a garbage bin in the crowded "Old Town" section of the city and was detonated by remote-control at mid-morning.

Chief Mahmoudi says police have no suspects and an investigation is underway.

The incident occurred five kilometers from a ceremony marking the disarmament of a local militia force, part of a U.N.-assisted program to demobilize militias across the country.

The militias were left in place after a U.S.-led war ousted Afghanistan's former Taleban regime in 2001. They were meant to provide security while the transitional government built up its national police and military forces.

Speaking to reporters in the capital, Kabul, U.N. special representative Jean Arnault would not say whether he thought the Herat bombing was related to the disarmament ceremony.

But he said some militia commanders may be seeking to hold onto power by staging violent incidents.

"In many, many countries, security incidents are created only for the purpose of demonstrating that this or that force is indispensable," said Mr. Arnault. "So there is no reason Afghanistan should be immune to this kind of tactics."

Some Afghan politicians have voiced concern that local militia commanders, described as semi-autonomous warlords by their critics, could try to influence Afghanistan's coming elections.

Mr. Arnault says that while the United Nations expects October's presidential election to be fair, he worries that militia commanders may have a negative impact on next spring's local and parliamentary elections if their units are not disarmed.

"It is indeed important to make sure that in the next eight months the climate, the environment for these parliamentary elections will not be what it would be today," he said.

While militia commanders have agreed publicly to comply with disarmament, U.N. officials have criticized some for seeking to delay the process. More than 10,000 local militia troops have surrendered their weapons and either joined the new national army or taken civilian jobs.

But as many as nine times that number of militia soldiers are believed to remain in Afghanistan.