A brutal reminder after Obama departs

The violence claimed at least seven lives, underscoring anew how what is supposedly the most secure Afghan city remains vulnerable to attacks that humiliate the government and its foreign backers while stoking fear among ordinary Afghans as U.S.-led combat forces withdraw.

"The attack was a message to Obama that the Afghan nation will not accept America's slavery," a Taliban spokesman, Zabihullah Mujahid, told McClatchy Newspapers by cellphone.

While Mujahid claimed responsibility on behalf of the Taliban, the U.S. ambassador to Kabul, Ryan Crocker, said that the Haqqani network, an extremist group based in Pakistan's tribal area, "cannot be ruled out" as the culprit. The United States blamed the group for a wave of attacks April 15 that lasted 18 hours and killed more than 60.

Shortly after Wednesday's violence, the Taliban announced on its website that its spring offensive would start Thursday.

It remains to be seen how potent the offensive will be. The insurgency has suffered serious blows since a U.S. troop buildup in 2010 that forced it to all but abandon conventional attacks.