Herat, Reuters—A co-ordinated attack by insurgents in western Afghanistan on Wednesday killed 34 civilians and 10 members of the security forces, officials said, one of the deadliest assaults in the country in more than a year.

Nine militants strapped with explosives stormed the governor’s compound in Farah province, bordering Iran, where a trial was being held for Taliban fighters, the governor’s spokesman Abdul Rahman Zhwandai said.

With the largest death toll from a single assault since 2011, the attack will increase concerns about how Afghan security forces will manage once NATO-led combat troops withdraw by the end of next year.

The Taliban claimed responsibility for the incident. The group’s spokesman Qari Yousuf Ahmadi said that the insurgents on trial were freed in the attack.

Ahmadi said the attack was aimed at government employees in Farah after the Taliban had “sent several warnings” to tell them to not work there.

Civilian casualties in the NATO-led war, now in its twelfth year, decreased in 2012 after rising for five years, according to the United Nations. More than 80 percent of civilian casualties are caused by insurgents.