Worshippers had gathered to observe religious day of Ashura on Tuesday.
Militants launch coordinated assault from four directions of the city.
Kabul government grants amnesty to militant accused of war crimes.
Mistakenly hitting Afghan forces or civilians has created friction for long.
Assault comes within hours after twin bombings killed 24 in the capital.
At least 13 people, including seven students, were killed and more than 40 others injured in an attack on the American University in the Afghan capital Kabul late on Wednesday.
Insurgents seized a city a few miles from province capital Lashkar Gah.
Incident follows an attack on a dozen foreign nationals in Herat last week.
On Monday Taliban struck a military and logistics facility in Afghan capital.
Heavily armed attackers storm into the compound after a powerful blast.
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