MOSUL (AFP) - A suicide bomber smashed a police base in northern Iraq and another blew up a patrol on Thursday, killing 20 people as a pro-government militia killed 15 suspected Al-Qaeda fighters, officials said. A man in an explosives-packed jacket blew himself up at police headquarters in Sinjar, a town on the road to Syria from the main northern city of Mosul. A hospital source said 17 people were killed and 30 wounded. Only hours before Thursday's attack, a suicide bomber targeted a group of police officers in Al-Gabat, a northern neighbourhood of Mosul, killing at least three people, including two policemen, and wounding 12 others. In Salaheddin province, a militia allied to US and government forces killed 15 suspected Al-Qaeda members at Tikrit. The surge in violence came as the US military announced plans to withdraw 4,000 more troops after saying the level of violence was at a four-year low. "The brigade played an integral role in establishing the conditions for long-term security in Iraq by reducing violence in Diyala province by 70 percent," a military statement said. US military spokesman Rear Admiral Patrick Driscoll said violence had declined 70 per cent since the surge. But he cautioned that Al-Qaeda still posed a serious challenge and had the ability to stage suicide bombings and fuel sectarian clashes.