WASHINGTON  - Security has improved in most parts of Afghanistan but insurgent sanctuaries in Pakistan and pervasive corruption pose ‘long-term and acute challenges’ in the war effort, a Pentagon report said Tuesday.It is also says thousands of tonnes of military equipment intended for the Afghan army and police is stranded in Pakistan, which for months has refused to reopen ground supply routes for Nato convoys despite high-level US pressure.Unless Pakistan reopens the routes, Afghan army units will face increasing shortages of equipment, particularly of vehicles, it adds. The issuing of the report to Congress came as US President Barack Obama made a surprise visit to Afghanistan, signing a partnership deal with Kabul pledging US aid after 2014, when all NATO combat troops are due to leave the country.The report, which covered the period from October 2011 to March 2012, says the Taliban and Al-Qaeda affiliates operating from sanctuaries in Pakistan and he limited capacity of the Afghan government remained biggest risks to making security gains durable.