By Hamid Shalizi   | KABUL (Reuters) –

A candidate for Afghanistan's September parliamentary elections was killed and 20 others wounded by a bomb planted in a mosque in Khost, officials said on Saturday.

Sayedullah Sayed, who also leads a small political party, was fatally wounded when a bomb detonated shortly before Friday's main prayers in the Ismail Khel of the province, provincial police chief Abdul Hakim Eshaaqzai said.

Violence is a major concern for Afghan voters who go to the polls on September 18 to choose from over 2,000 candidates standing for around 250 seats.

"We don't want to blame anyone for this attack as the investigation is still going on," Eshaaqzai told Reuters.

The Taliban, who frequently deny responsibility for attacks that target mosques, said the bomb could have been the work of Sayed's rivals.

The head of the provincial hospital, Amir Badshah Rahmatzai, said nearly 20 people were brought to the hospital along with Sayed, who lost both of his legs in the attack and later died.

Taliban insurgents carried out more than 130 attacks on polling centres and other targets during presidential elections last year, killing nine civilians and 14 Afghan security forces.

In other violence, four members of NATO-led International Security Assistance Force, (ISAF) were killed in an Improvised Explosive Device (IED) attack in southern Afghanistan, the alliance in a statement.

NATO did not disclose nationalities of the casualties nor the exact location of the incident.

Elsewhere, dozens of Taliban and "foreign fighters" attacked a district in mountainous Nuristan province near the Pakistani border, provincial governor Jamaluddin Badar said.

Barge Matal district, which has already changed hands twice this year, has been the target of raids from Pakistani Taliban since the withdrawal of Americans troops last year.

Badar said the government troops, who are also backed by armed villagers, are outnumbered by the insurgents but expect reinforcements from nearby provinces soon. NATO troops only provide air support, he said.

(Additional Reporting by Elyas Wahdat in Khost; Writing by Hamid Shalizi; Editing by David Fox)