Kabul, Afghanistan (CNN) -- NATO-led troops fired on a bus in southern Afghanistan on Monday morning, killing four civilians and wounding 18 others, officials said.
The wounded included women and children, said Zalmai Ayoubi, the spokesman for Kandahar province where the shooting occurred.
The bus was headed from Kandahar to Oruzgan province when troops with the International Security Assistance Force (ISAF) asked the bus driver to stop.
The bus driver did not recognize the troops because it was still dark, said Ayoubi.
The troops then fired on the bus, he said.
Soldiers later took some of the wounded civilians to an ISAF clinic.
Interior ministry spokesman Zemeri Bashary confirmed the shooting. ISAF officials also confirmed the incident and released a statement, saying troops tried to warn the approaching vehicle to stop by shooting up flares and then shot at the vehicle after it did not stop. The coalition troops then thought the approaching vehicle was a threat and fired at it, the statement said.
"ISAF deeply regrets the tragic loss of life in Zhari district this morning," the statement said.
Meanwhile, gunmen entered a school near the national security building in Kandahar where Taliban prisoners are held.
In an ensuing gun battle with police, two would-be suicide bombers were killed. The gunfire exchange also wounded two security officials and a teacher at the school.
Journalist Matiullah Mati contributed to this report