Israeli airplanes carried out several raids in the Gaza Strip on early Friday, targeting open spaces and a training site for the Islamic Hamas movement, security sources and witnesses said. There have been no casualties in the four airstrikes that the Israeli army started around midnight. The targeted places were said to have been used by Palestinian militants to produce, store and fire rockets into Israel. An Israeli army spokesman told Xinhua that the first airstrike hit a building 'used to fire rockets from.' The three other raids hit open lands in Central and Southern Gaza Strip. The spokesman, Avichai Adraee, threatened that the Israeli army will continue its operations as long as Palestinian fighters launch missiles. The Israeli raids took place hours after a rocket landed in port city Ashkelon in southern Israel. The rocket caused no injuries or damage. On Thursday morning, Palestinian militants fired mortars at Israeli army posts near Gaza Strip's commercial crossings and Israel closed one crossing in response. Ten mortar shells landed near Kerem Shalom crossing, which Israel uses to deliver limited amounts of humanitarian aid and fuel to the besieged territory, and a military site near the closed Kissufim passageway in southeast Gaza Strip. Following the incident, Radio Israel said that Kerem Shalom crossing was sealed off. Also on Thursday, Israeli planes dropped flyers in northern Gaza Strip, warning residents to stay away from an electric fence between the Hamas-controlled coastal Strip and Israel. The leaflets, with telephone numbers and email addresses on them, urged people to inform the army of any activities of the Palestinian militants in the border area. 