(Adds army investigating girl's shooting, paragraph 8) By Nidal al-Mughrabi GAZA, April 21 (Reuters) - Israeli forces killed at least six Palestinians on Saturday, including one in an air strike in the Gaza Strip, in the worst flare-up in Israeli-Palestinian fighting in months. The Israeli army said the air strike, only the second in Gaza since a November truce, targeted militants who had fired makeshift rockets at the Israeli border town of Sderot, hitting a house. A top aide to Palestinian President Mahmoud Abbas said the Israeli actions jeopardised Abbas's efforts to expand the fragile truce from Gaza to the occupied West Bank as part of a U.S.-led peace push. In the most deadly incident, an Israeli undercover unit killed three armed militants while they were driving in the northern West Bank city of Jenin, Palestinian security sources said. In a village near Jenin, a Palestinian policeman was also shot and killed by Israeli forces, Palestinians witnesses said. They said the policeman was shot when he peered out of his window and had not been involved in any fighting in the area. The Israeli army said its troops "noticed an armed militant firing from the top of a building and identified hitting him." Later in the day, a 17-year-old Palestinian girl was shot dead by Israeli troops as she stood at her window in Jenin refugee camp, Palestinian medical workers said. The army issued a statement in which it said it had launched an investigation into the girl's shooting and said that the findings would be brought before the military advocate general for consideration whether to charge anybody in the matter. Palestinian militants in Gaza responded to the West Bank killings by firing at least three rockets at Sderot. An Israeli ambulance service said two people in the Israeli town which lies close to the border with Gaza were treated for light injuries. The Islamic Jihad militant group said a 45-year-old man was killed in the Israeli air strike. The group described him as a civilian but said he was riding in a car with two militants, who were seriously wounded. "The Israel Air Force fired at a rocket-launching cell which launched the Qassam rockets at the Israeli town of Sderot," an Israeli army spokesman said. An Israeli ambulance service said two people had been taken to hospital and were being treated for light injuries after one of the rockets hit a house in Sderot. Local residents said four others, including a pregnant woman, were being treated at the scene for shock. Palestinian Prime Minister Ismail Haniyeh of the Islamist faction Hamas condemned the killings in a statement. "These crimes are new proof on the brutality of the occupation (Israel), its pursuit of liquidation and is a futile attempt to break the will of the Palestinian people and push them to surrender," the statement read. Haniyeh urged Arab countries to help break an international aid embargo on the Palestinian government and not to normalise relations with Israel. A top Hamas leader in the Gaza Strip, former foreign minister Mahmoud a-Zahar, seized on the Israeli actions and vowed the militant group, which leads the Palestinian government, would continue to fight Israel until "the liberation of Palestine, all of Palestine." Abbas aide Nabil Abu Rdaineh, said: "This is dangerous aggression and it will lead to instability at a time when the Palestinian Authority is making great efforts to maintain a truce. We urge the international community to intervene immediately to stop Israel's aggression." (Addtional reporting by Wael al-Ahmad in Jenin, Ori Lewis and Ari Rabinovitch in Jerusalem and Yehuda Peretz in Sderot)
