A US military chief has issued a warning that the border region between Afghanistan and Pakistan remains the epicentre of terrorism during an unannounced visit to the NATO-led mission. Adm Mike Mullen reiterated that Pakistan was still a safe haven harbouring Al-Qaeda despite the death of Osama bin Laden at the beginning of May. Speaking to NATO TV, he said: "Pakistan still is home to many terrorists and other terrorist organisations like the Lashkar-e-Taiba (LeT) and Haqqani network. "And, clearly, those who live in Quetta come here with a goal of overturning the government of Afghanistan." Twelve Afghan police officers and one civilian were killed when a suicide bomber attacked the police headquarters in the provincial capital, Lashkar Gah, where British Task Force Helmand is also based. But Adm Mullen said he was confident that the transition would continue with the International Security Assistance Force (ISAF) working alongside Afghan counterparts. "We recognise the challenges posed by the some of the recent spectacular attacks and assassinations. We are doing all we can with the Afghan security forces to prevent those in the future. "We see a situation in the future where many of the Taliban leaders have either been taken off the battlefield or have returned to the safe havens in Pakistan. We see weakness in the Taliban's tactics in some areas," he asserted. Adm Mullen also stressed that the withdrawal of 33,000 US troops - announced by President Obama in June - would not put the NATO-led mission at risk. Ten thousand American soldiers are scheduled to leave Afghanistan by the end of the year with the remaining 23,000 troops withdrawn by September 2012.