A huge bomb blast Friday at one of the biggest mosques in western Afghanistan killed at least 18 people, including an influential imam who earlier this year called for those who commit "the smallest act" against the government to be beheaded.

Images posted on Twitter showed what appeared to be blood-stained bodies scattered around the compound of Gazargah Mosque in Herat city.

Violence has largely declined since the Taliban returned to power last year, but several bomb blasts -- some targeting minority communities -- have rocked the country in recent months, many claimed by the jihadist Islamic State (IS) group.

At least 18 people were killed and 23 wounded in Friday's blast, said Hameedullah Motawakel, spokesman for the governor of Herat province, in a text message to media.

Government spokesman Zabihullah Mujahid confirmed Mujib ur Rahman Ansari, the imam of the mosque, was among the dead.

"A strong and courageous religious scholar of this country was martyred in a brutal attack," he said on Twitter.

Ansari was an influential cleric known for his fiery speeches.

In July, during a religious gathering in Kabul, he strongly defended Afghanistan's new Taliban rulers.

"Whoever commits the smallest act against our Islamic government should be beheaded," he said.

"This (Taliban) flag has not been raised easily, and it will not be lowered easily."

Before the Taliban returned to power in August last year, Ansari was known for his tirades against the previous US-backed governments.

Ansari is the second pro-Taliban cleric to be killed in a blast in less than a month, after an earlier suicide attack targeted Rahimullah Haqqani at his madrassa in Kabul.

Haqqani was known for angry speeches against IS, who later claimed responsibility for his death.

He had also spoken in favour of girls being allowed to attend secondary school, despite the government banning them from attending classes in most provinces.

Several mosques across the country have been targeted this year, some in attacks claimed by IS.

At least 21 people were killed and dozens more wounded on August 17 when a blast ripped through a mosque packed with worshippers in Kabul.

IS has primarily targeted minority communities such as Shiites, Sufis and Sikhs.

While IS is a Sunni Islamist group like the Taliban, the two are bitter rivals and greatly diverge on ideological grounds.

Government officials claim that IS has been defeated but experts say the group is the main security challenge for the country's current Islamist rulers.