IDF heavy guns pounded the northern Gaza Strip on Sunday, killing a Palestinian Authority police officer and wounding at least 16 others as the army escalated its retaliation to Kassam rocket fire, while accusing the new Hamas-led PA government of not reining in terror groups. IDF troops also saw action in the West Bank and shot dead terror suspects in both Nablus and Bethlehem. Fifteen Palestinians were killed over the weekend, including the young child of a senior bombmaker, in IAF air strikes on Kassam launch sites and terrorist training camps in the Gaza Strip. On Sunday, the IDF kept up the severe pressure and threatened to invade Gaza if the Kassam fire did not come to a complete stop. "In the end there will not be Kassam fire," a senior officer asserted. "And our operation might in the end include returning to parts of the Gaza Strip from which they fire rockets... we will continue escalating our actions." But despite the IDF operations and nonstop artillery barrages, terror cells still succeeded Sunday in firing at least five Kassams. Two of them, the army said, landed in PA territory, while a third landed in an open field near a Negev kibbutz. Defense Minister Shaul Mofaz briefed the cabinet Sunday morning on the IDF's weekend activities in the Gaza Strip, saying it attacked Kassam cells, areas where the rockets were launched, offices serving terrorist groups and their training camps. "We will do everything necessary at all hours of the day to reduce the Kassam fire," he said. The terrorists' motivation to carry out attacks, Mofaz said, had increased significantly over the last few months. He said that there were currently 11 concrete warnings of attacks, as well as partial information regarding 70 others. Mofaz said that as a result of the increase in the number of attacks and warnings, as well as Hamas taking over the PA government, Israel would continue to take an "uncompromising policy" toward the terrorist organizations and "anyone who threatens our citizens." Meanwhile Sunday, troops from the Duvdevan Battalion killed senior Fatah Tanzim fugitive Jabar Akhras during an arrest operation east of Bethlehem. Armed with a Kalashnikov rifle, Akhras rushed the soldiers and was shot. Earlier in the day, troops shot and killed a Palestinian in Nablus. Akhras, security officials said, was responsible for the murder of two Nahal Brigade soldiers, Shlomo Bilski and Shaul Lahav, at the roadblock on the tunnel road between Gush Etzion and Jerusalem in November 2003. He was also involved in planning other attacks, and worked on attempts to build Kassam rockets in the West Bank. Also Sunday, the Shin Bet (Israel Security Agency) revealed that it had arrested Azzam Zin a-Din - a Fatah operative from the Nablus area who admitted to murdering Eldad Abir, shot dead at a gas station near the settlement of Migdalim on March 1. A-Din, the Shin Bet said, was arrested on March 7 as he tried to flee into Jordan. Hamas's military wing condemned the "dangerous escalation" in Gaza and vowed revenge. "We warn the government of this monstrous entity against committing more crimes, because this will provoke more destruction and escalated military attacks against them and their people," it said in a statement posted on the Hamas Web site. Palestinians said that the police officer killed, Yasser Abu Jarad, 28, was trying to evacuate colleagues from a makeshift military post when a shell hit his car in Beit Hanun. Earlier in the day, the army dismissed a report that Islamic Jihad had offered a week-long cease-fire in exchange for a halt to the IDF Gaza operation, dubbed "Southern Arrow." But later in the day, Islamic Jihad denied ever offering the cease-fire and vowed to step up its attacks. "Islamic Jihad is going to escalate its attacks on the Zionist entity by all possible means," Khader Habib, an Islamic Jihad leader in Gaza, said. "We are going to teach the government of Tel Aviv a lesson they are not going to forget." Islamic Jihad is responsible for many of the 40 rockets fired since the beginning of April. Abu Abdullah, an Islamic Jihad spokesman in Gaza, said the group would fire Kassams and longer-range Katyushas, capable of hitting Ashkelon and sensitive facilities there. In late March, Islamic Jihad became the first group to fire a Katyusha from Gaza. There will be "no truce with the occupation while there is an open war," Abu Abdullah said. "We reaffirm there will be an immediate response." AP contributed to the report.