Officials in southern Afghanistan say a roadside bomb blast hit a passenger bus Sunday, killing five civilians, including women and children.

Authorities say the early morning blast wounded at least 18 other people in Ghazni province. The bus was traveling from Kabul to the southern city of Kandahar.

One of the wounded told Reuters news agency that shortly before the explosion, NATO soldiers nearby warned the driver against going ahead but he did not listen.

No one has claimed responsibility for Sunday's bombing, but placing improvised explosive devices along roads is a common insurgent tactic.

According to the United Nations, anti-government forces are responsible for the vast majority of civilian casualties in Afghanistan.

Also Sunday, an insurgent wearing an Afghan police uniform shot and killed three NATO service members in southern Afghanistan. NATO is investigating the incident and did not release the names or nationalities of the dead.

U.N. special representative for Afghanistan Jan Kubis said in May that insurgents were responsible for nearly 80 percent of the 579 civilian deaths in the first four months of this year. That is about the same percentage as last year. For the same four months, NATO and Afghan forces were responsible for nine percent of civilian deaths — a drop of five percent from the previous year.

The overall number of civilian casualties dropped by more than 20 percent during the first four months this year as compared to the same period in 2011.

This was the first reduction since the United Nations started tracking the figures five years ago.

Last year, a record 3,021 civilians died as insurgents increased their use of indiscriminate weapons, including roadside bombs and suicide bombers.