Twenty people were killed and 50 injured in a suicide attack on the Kabul office of Amrullah Saleh, a vice presidential candidate and the running mate of Afghanistan President Ashraf Ghani, officials told Reuters on Monday. The attack, in which Saleh received minor injuries, occurred on Sunday as the first day of campaigning for the September elections began.

No group has claimed responsibility for the incident yet. Afghan security forces have reportedly killed three militants who had entered Saleh’s office after the suicide attack, The Guardian quoted Interior Ministry Spokesperson Nasrat Rahim as saying. In a six-hour-long operation, over 150 civilians had been rescued, the official said.

United States Special Representative Zalmay Khalilzad condemned the attack, describing it as “grotesque and a clear act of terrorism”. “We condemn it in the strongest terms,” he said. “The perpetrators should be brought to justice. We stand with Afghanistan in the fight against terror.”

Presidential elections, which have already been delayed twice this year, are scheduled in the country for September 28 but the overall security in Afghanistan has taken a turn for the worse with the Taliban and Islamic State groups launching frequent attacks on the Afghanistan forces or government offices. Last week, three bomb blasts hit Afghanistan’s capital Kabul, killing at least 15 and injuring dozens.

The attacks came amid ongoing talks between members of the Taliban and the United States’ diplomats to figure out a schedule for withdrawal of foreign forces from the country. The talks are expected to resume early in August, according to Reuters. On Sunday, Ghani insisted that peace was being restored in the region, however, he has been excluded from the discussions with the Taliban as the latter has refused to negotiate with the government, which it claims is a foreign-appointed puppet.