North Korea\'s air force has sharply increased sorties since mid-May, with some fighter jets flying close to South Korea\'s airspace, Yonhap News Agency reported Wednesday, citing a military source. \"Up to 50 sorties have been made a day, and two to three of them approached close to the tactical action line (TAC),\" the source was quoted as saying. The TAC is an imaginary line that the South established about 20-50 kilometers north of its airspace as a heads up for possible provocations. South Korean fighter jets are required to scramble if North Korean aircraft approach the line. On Tuesday, one North Korean fighter jet approached the TAC, prompting four South Korean jets to scramble, the source said. \"The number of flights is believed to have increased as the North\'s air force has been under its summer combat inspections,\" the source said. \"We are making thorough preparations for the possibility of airborne provocations.\"