A top female police officer has been shot dead in Afghanistan's Helmand province, officials say.

Lt Islam Bibi was ambushed by unknown attackers as she left her home in the provincial capital, Lashkar Gah, a spokesman for Helmand police said.

The commander of 32 female police officers, Lt Bibi, 37, was known as a role model for other women in the conservative province.

Her death came as four girls were also killed in a roadside blast in Helmand.

Lt Bibi was on a motorbike alongside her son-in-law when she was wounded in the gun attack. She later died in hospital, officials say.

Like most Afghan women in rural areas of Afghanistan, Lt Bibi had struggled to work outside the home, says the BBC's Bilal Sarwary in Kabul.

Meanwhile, the four girls, said to be aged between seven and 12, were attending a wedding in Lashkar Gah and had gone to collect water from a stream when the explosives detonated.

Last month Afghan forces assumed security responsibility for the whole country ahead of Nato's departure. Improvised explosive devices (IEDs) are one of their biggest challenges.

Helmand is one of Afghanistan's most volatile provinces.

Correspondents say that civilian casualties in the south have increased recently, and many of those are due to the use of improvised explosives by the militants.