An Afghan man receives treatment after a bomb attack in Kabul, April 19

KABUL — A Taliban suicide bomb and gun assault on a government security building during Tuesday morning rush hour in central Kabul killed at least 28 people and wounded more than 320, in the deadliest single attack in the Afghan capital since 2011.

President Ashraf Ghani condemned the assault “in the strongest possible terms” in a statement from the presidential palace, located only a few hundred meters away from the scene of the blast.

The insurgency led by the Afghan Taliban has gained strength since the withdrawal of most international combat troops at the end of 2014, and the extremist group is believed to be stronger than at any point since it was driven from power by U.S.-backed forces in 2001.

With nerves on edge in Kabul, a second explosion hit the city in the evening but no serious casualties were reported from the blast, which was caused by an improvised device, according to Interior Minister spokesman Sediq Sediqqi.

Police chief Abdul Rahman Rahimi said civilians and members of the Afghan security forces were among those killed and wounded in the morning blast, when a suicide car-bomber blew himself up outside the wall of a National Directorate of Security (NDS) office.

Witnesses described chaotic scenes after the blast.

“I was here when a huge explosion happened,” said Amir, who works in a nearby restaurant. “I saw three boys with severe head injuries. My uncle was injured and my brother is missing, I don’t know what happened to him.”

It was the worst single militant strike in Kabul since 2011, when about 60 people died in a suicide blast outside a mosque, and will reinforce concerns in Afghanistan and the West that the country is being dragged into a worsening spiral of violence.

Rahimi said one attacker had tried to slip into the NDS building through a destroyed wall after the blast, but he was discovered and killed.

The Taliban said on their Pashto-language website that they had carried out the suicide bombing on “Department 10”, an NDS unit responsible for protecting government ministers and VIPs.

They said a suicide car bomber blew up the main gate at the front of the office, allowing other fighters, including more suicide bombers, to enter the heavily guarded compound.