KANDAHAR, Afghanistan -- A bomb blast near a police station in southern Afghanistan on Tuesday killed three people and wounded 17, while a separate bomb killed one person and wounded five in the north, officials said.

The first blast took place early Tuesday in the southern city of Kandahar, said Samim Khpalwak, spokesman for the provincial governor. Authorities initially said a car bomb caused the explosion, but later said it was caused by a bomb planted near the police station.

Qayyum Pokhla, the head of Kandahar's regional hospital, said the blast killed two police officers and a woman, and wounded another 17 people.

Kandahar province is the heartland of the Taliban insurgency, and the city was the capital of the extremists' government from 1996-2001.

Sayed Sarwar Hussaini, spokesman for the provincial police chief in Kunduz province, said at least one civilian was killed when a remotely detonated bomb attached to a bicycle exploded as a police vehicle was driving past. Five other civilians were wounded in the attack, which occurred in the provincial capital of the same name, Hussaini said.

No group immediately claimed responsibility for either attack.

Attacks on Afghan security forces have intensified in recent weeks as the insurgents mount their annual summer offensive.

Meanwhile, 19 Afghan miner clearers who were abducted on Sunday have been freed by their captors, Gen. Zelmai Oryakhail, provincial police chief of Paktia province, said Tuesday.

Tribal leaders and villagers in the Zurmat area where the mine clearers had been working mediated for their release, he said, adding that no ransom was paid. Oryakhail said the kidnappers were probably Taliban-affiliated insurgents. Officials had earlier said that 12 mine clearers were taken.