Iraqi security forces backed by US troops are pressing ahead with a major offensive in Diyala province, an insurgency stronghold north of Baghdad. Troops sealed off the provincial capital Baquba on Tuesday and have been searching for suspects and weapons. Officials said 35 wanted insurgents had been detained by the second day of the operation involving some 50,000 troops. Hot weather and the inhospitable terrain are making progress extremely difficult, military officials say. House-to-house searches were currently focused on Baquba itself, but would be extended to rugged areas near the Iranian border, regional council head Ibrahim Bajilan said. The crackdown would take about two weeks "and then law will be imposed in all Diyala", he said in an interview with AP news agency. Diyala province has proved to be one of the most stubborn areas to pacify following the success of calming former insurgency hotbeds such as Anbar province to the west of Baghdad. This latest operation was generally expected but its timing was kept secret, and army and police units were brought up from Baghdad unannounced. Correspondents say there was no significant sign of the violence and destruction that marked operations in the area last year. 'Exceptional co-operation' Defence Ministry spokesman Muhammad al-Askari said roads out of Diyala had been blocked to prevent al-Qaeda-inspired insurgents from slipping away to other regions. "We are not fighting an organised army but we are fighting terrorist groups hiding either in palm tree groves or among civilians," Maj Gen Askari said. The US military have been providing intelligence, logistical and military support, but Iraqi forces were in the front line. Correspondents say Iraqi units are displaying a readiness to take over security responsibility from the US, which could allow an eventual reduction of US forces in Iraq. Diyala's population is divided between Shia and Sunni Muslims and it controls key supply routes between Baghdad and northern cities as well as having a long border with Iran. A US military spokesman said the co-operation with the local population in the current operation was "exceptional". In the past the Sunni community has accused the Shia-dominated Iraqi security forces of discrimination by failing to secure Sunni areas. But correspondents say the tables have been turned since the emergence of local anti-al-Qaeda groups known as Awakening councils which have been backed by the US and Iraqi security forces.