Mohammed Rafique Shahrzad, a hospital official in Herat city, confirmed that the bodies of 20 victims killed by the explosion were brought to the main hospital in the provincial capital soon after the blast.
The assault on the Jawadya mosque in Herat, which is close to Afghanistan's border with Iran, came a day after the Islamic State in Iraq and Syria (ISIS) group claimed a deadly attack on the Iraqi embassy in Kabul.
Other local reports suggested a combined explosion and gunfire attack.
An explosion at a mosque in the Afghan city of Herat has killed at least 15 people and injured dozens more.
Hospital authorities in Herat said all the dead and wounded were worshipping in the mosque when a suicide bomber blew himself up.
"More than 30 wounded have so far been brought to the hospital", Rafeeq Shirzai told AFP.More news: Man United in sponsorship deal talks with Tinder
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Afghan interior ministry spokesman Najib Danish initially said he thought it was a vehicle bomb and tweeted that there was a possibility that it was a improvised explosive device.
A spokesperson for the regional governor told the BBC that the suicide attacker had opened fire inside the mosque before detonating his explosives.
The death toll is expected to rise, according to local officals.
No-one immediately took responsibility for the attack on the Jawadia Mosque in Herat.
Areas dominated by Shia Muslims in Afghanistan have been hit by attacks repeatedly in the past year, by both IS and the Taliban.