KABUL, Afghanistan - A suicide bomb blast killed a provincial leader and at least 12 other people Monday in northern Afghanistan, the latest in a step-up of attacks by Taliban insurgents as warm weather returns.

The attack occurred in the city of Pul-i-Khumri at the offices of Mohammad Rasoul Mohseni, chief of the Baghlan provincial council, said Mahmood Haqmal, a spokesman for the Baghlan governor's office.

The bomber came on foot to the building, pushed through Mohseni's bodyguards, and detonated his explosives-filled vest as he entered the compound, Haqmal said.

Mohseni, six bodyguards, and six civilians were killed. Nine people were injured, Haqmal said.

Afghan President Hamid Karzai called the attack "a cowardly act against all Islamic teachings and human values." The Afghan Taliban claimed responsibility for the blast.

Insurgent attacks have increased in recent weeks. The Taliban announced the start of its spring offensive in April, when the melting of snow blocking paths and mountain passes enables militants to launch attacks.

A suicide car bombing Thursday in the capital, Kabul, targeted a NATO convoy, killing at least 15 people. Two of the dead were U.S. soldiers, and four were American civilian contractors working for DynCorp, a U.S. military contractor in Afghanistan to train Afghan security forces. The others killed were Afghan civilians.

On Friday, militants in the far western province of Farah killed a police chief known to be an outspoken critic of the Taliban. Abdul Ghani was shot to death by gunmen on motorcycles as he pulled out of his driveway.