Mehdi Hadid, a lawmaker from Herat, told The Associated Press that he estimated at least 100 had been killed and said he saw bodies scattered throughout the mosque. "They are both dead", he added.
August 1: An explosion in front of Jawadia Mosque in Dehr Abad area in Herat city has killed around 20 people.
He reported seeing a body torn to pieces at the entrance, possibly that of the attacker, while others were lying in pools of blood inside, some still crying and moving.
About 30 people are reported to be wounded, and local officials say the death toll is expected to rise.
According to them, a suicide bomber opened fire on worshipers inside the mosque before detonating his explosives. The report has yet to be confirmed by local sources in Afghanistan.
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No one immediately claimed responsibility for Tuesday's attack on Herat's Jawadia Mosque.
Herat, on the border with Iran, is considered one of the most peaceful cities of Afghanistan.
Shiite Muslims have remained a constant target of militant attacks across various parts of the country over the past few months.
The attack comes a day after an attack on the Iraqi embassy in Kabul, which was claimed by the ISIS, killed two people.
But experts have previously questioned whether there are direct links between the group's local affiliate Islamic State Khorasan Province (IS-K) and the central IS command.