Jane Onyanga-Omara, and John Bacon

USA TODAY

Explosions and gunfire ripped through the Afghan capital of Kabul on Tuesday as the Taliban spring offensive's most brutal assault yet left dozens dead and wounded hundreds more.

The United Nations special representative for Afghanistan said the attack "may amount to war crimes."

The streets were bustling during morning rush when a suicide bomber detonated a vehicle laden with explosives near the compound of a government secret service unit whose role is to protect VIPs, Kabul police chief Abdul Rahman Rahimi said. Militants then rushed into the building, sparking a gunfight with security forces that lasted more than two hours, local broadcaster TOLOnews reported.

At least 28 people were killed and more than 300 wounded, Rahimi said. He added that most of the victims were civilians, including women and children, and that many of the wounded were in critical condition.

Interior Ministry spokesman Sediq Sediqqi said at least one of the attackers was killed in a gun battle, the Associated Press reported.

President Ashraf Ghani condemned the attack, which started about 9 a.m. local time. Atta Mohammad Noor, the acting governor of northern Balkh province, encouraged residents to launch a massive public uprising against the Taliban, Pajhwok news service reported.

Taliban warn of attacks in new fighting season

Taliban spokesman Zabihullah Mujahid claimed responsibility for the assault. The Taliban has stepped up attacks since announcing the start of its spring offensive dubbed "Operation Omari" a week ago. Taliban founder Mullah Mohammad Omar is believed to have died of illness in 2013. The terror group didn't acknowledge his death until last year.

Tadamichi Yamamoto, the secretary-general’s deputy special representative for Afghanistan, said the attack demonstrated the Taliban's "complete disregard for the lives of Afghan civilians."

He also said the attack, “ in circumstances almost certain to cause immense suffering to civilians," might be considered a war crime.

Police commander Obaidullah Tarakhail told AP he couldn't hear or see for 20 minutes after the initial blast.

“This was one of the most powerful explosions I have ever heard,” he said. "All around was dark and covered with thick smoke and dust."

The U.S. Embassy in Kabul condemned the "senseless act of violence" that it said underscores the brutality of the Taliban.

"Afghanistan deserves peace and security, not attacks that victimize parents taking their children to school, workers on their morning commute, and people who have stepped forward to help defend their fellow citizens," the embassy said in a statement.

Gen. John Nicholson, the commander of NATO's Resolute Support mission in Afghanistan, also condemned the attack, saying it "shows the insurgents are unable to meet Afghan forces on the battlefield and must resort to these terrorist attacks."

"Resolute Support expresses its deepest condolences to those affected by this contemptible act of violence. We will continue to assist our Afghan partners in achieving a sustainable peace as well as security for the people of Afghanistan," he added.