The girls, aged 10 to 12 years, had been sent by their families to a river to fetch some water for a wedding that was being held in a home on the outskirts of the provincial capital of Lashkar Gah, Helmand governor's spokesman Ummar Zawaq said.
The bomb went off alongside a footpath through a field the girls had taken to make their way back home, Zawaq said.
Also in Lashkar Gah today morning, two gunmen on a motorcycle opened fire on a female police inspector who was being taken to work by her son-in-law on his motorcycle, Zawaq said. Inspector Islam Bibi was rushed to the hospital with her wounds but died there about an hour later, he said. Her son-in-law was also wounded.
Meanwhile, the NATO-led coalition said one of its service members was killed in fighting Thursday in western Afghanistan. It gave no further details.
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Violence this year has matched its worst levels in nearly 12 years of war. According to the most recent figures from the Interior Ministry, 180 civilians were killed across Afghanistan last month.
In the same time frame, 299 police officers and 753 militants were killed, the ministry said. The Defense Ministry did not have similar figures for the Afghan army.