(CNN) -- Trucks carrying supplies to NATO troops crossed from Pakistan into Afghanistan for the first time in seven months on Thursday after Islamabad agreed to reopen routes, officials said. The four trucks, under heavy security, crossed the border from Chaman in Pakistan's Balochistan province. Because Afghanistan is landlocked, many supplies for NATO-led troops fighting Islamic militants have to be trucked in from Pakistan. On Tuesday, Islamabad decided to reopen the crucial supply routes shut down on November 27, a day after coalition forces mistakenly killed 24 Pakistani troops. The incident plunged U.S.-Pakistan relations to an all-time low. The Pakistani routes offer a shorter and more direct route than the one NATO has been using since November that went through Russia and other nations, avoiding Pakistan altogether.