"Based on our initial information two terrorists were involved, one of them wearing a suicide vest who detonated himself while the second one was armed with a rifle".
Shias, a minority of around three million in overwhelmingly Sunni Afghanistan, have regularly been targeted by IS jihadists over the last year.
The Secretary-General urged Afghan authorities to take all possible measures to bring perpetrators to justice, reiterating his call on regional and international cooperation in the face of terrorism and violent extremism in order to secure a safer world.
"The death toll has risen to 29 killed and 63 wounded".
Provincial spokesman Jilani Farhad says that to reduce the chance for more attacks, a planned Shiite protest to denounce the attack will be held just before the burial on Wednesday afternoon, rather than at a separate time and location.
"Fanning terror and sectarian violence against a specific community is abhorrent and those responsible must be brought to account", said Tadamichi Yamamoto, the UN Secretary-General's Special Representative for Afghanistan.
The attack started when one suicide bomber began firing on worshippers inside the mosque before blowing himself up, said Abdul Ahad Walizada, spokesman for the police chief.
The Herat attack was the latest in a series of assaults on Afghanistan's minority Shiite population.
"The mosque is God's home and we think it's the safest place in the world, but people were butchered there as well".
"The government cannot even ensure security of worshipers in the mosque".
He said at least 29 people had been killed, and another 64 were injured. It is not clear what the intended target was. "When I heard a loud bang, I closed my shop and walked towards the area where the attack took place", Mohammadi told Al Jazeera.
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