HERAT (AFP) - NATO troops on Thursday made Afghan security forces responsible for the city of Herat, the country's western capital where two British men were reportedly arrested. Herat is the fourth of seven areas to transition to Afghan control this week, but critics say the timetable is politically motivated and not reflective of Afghan abilities to ward off the Taliban with violence at a 10-year high. Underscoring the threat, a suicide bomber killed four people on Wednesday in the north's Mazar-i-Sharif, considered one of the safest cities in Afghanistan and due to be transferred on Saturday. In Herat, a ceremony took place at NATO's International Security Assistance Force headquarters in the city, attended by the defence minister, other senior Afghan officials and Italy's economic development minister Paolo Romani. Italy is in charge of coalition efforts in Herat. "We don't want a parallel security force. The ANA (army), ANP (police) and our intelligence forces are prepared to defend our country and crush the enemy," said Ashraf Ghani, head of the national transition authority.