IDF reservists took up responsibility for areas in northern Gaza on Monday, enabling regular-service infantry units to push deeper into Gaza City as defense officials said that Israel would continue expanding the ground operation until it received assurances that weapons smuggling would stop along the Philadelphi Corridor. They said that thousands of reservists entered Gaza and thousands of others had been called up. The IDF was "tightening" its grip on Gaza City and was sending troops deeper into the built-up urban centers where Hamas fighters are believed to be holed up, the officials said. The IDF has also been operating in southern Gaza. Defense officials said that at least 150 smuggling tunnels were still believed to be intact in the Philadelphi Corridor after the IAF destroyed some 200. Hamas was still using the tunnels and trying to smuggle weaponry into Gaza, although in much smaller quantities than before the operation, they noted. A soldier was seriously wounded and three others lightly wounded on Monday night in an apparent friendly fire incident between IDF paratroopers in northern Gaza. Meanwhile Monday, the IDF clashed with Hamas gunmen throughout northern Gaza and attacked a mosque near Al-Atatra, from where Hamas had fired at troops and was storing a large weapons cache. In total, the IDF has bombed close to 10 mosques since the beginning of Operation Cast Lead on December 27. IDF paratroopers discovered and destroyed a terrorist smuggling tunnel on Monday evening, some 300 meters from the Gaza border. The IDF said that Hamas had planned to use the tunnel to infiltrate Israel and carry out a terror attack. In another incident, the IDF said that soldiers identified a number of gunmen and opened fire at them. Troops later searched the area and uncovered a large amount of weaponry including anti-aircraft missiles, mortar shells and machine guns. IDF ground forces continued to battle Gaza gunmen, while the IAF struck Hamas installations in the northern Strip and in Gaza City. Four soldiers were lightly wounded in the fighting and evacuated to Beersheba's Soroka Hospital. A reservist who was a conscientious objector was sentenced to two weeks in military jail for refusing to serve in Gaza. Meanwhile, according to an army estimate Monday, slightly over 900 Palestinians have been killed since Operation Cast Lead began. Based on intelligence and information obtained by the Gaza Coordination and Liaison Administration, the IDF has determined that at least 400 of those killed are known Hamas operatives. The IDF believes that among the remaining 500, at least half are Hamas operatives. The IAF hit over 25 targets throughout Gaza on Monday, including eight groups of gunmen, two mortar launchers, four launching pads, three buildings and two vehicles driven by Hamas terror operatives. Palestinians reported that five people were killed in one of the air strikes, including two women and a child. They said that several others were also wounded. Meanwhile, The Jerusalem Post has learned that a shipment of US ammunition bound for Israel and not linked to Operation Cast Lead might be delayed until after the fighting stops. The shipment was scheduled to dock in Ashdod in mid-to-late January but, as a result of the fighting in the area might be postponed. The postponement is believed to be for logistical reasons concerning the fact that the port lies within rocket range from Gaza, and not political ones. Reuters reported that the US military sought to hire a merchant ship to deliver the ammunition to Israel this month from a port in Greece, but that the Pentagon said the shipment was not linked to the conflict in Gaza. A Pentagon spokesman told the news agency the ammunition was for a US stockpile in Israel. The US military pre-positions stockpiles in some countries in case it needs supplies at short notice. Reuters quoted Air Force Lt.-Col. Patrick Ryder, a Pentagon spokesman, as saying "The delivery of ammunition is to a pre-positioned US munitions stockpile in Israel in accordance with a congressionally authorized 1990 agreement between the US and Israel." US officials told the Post that the shipment was coordinated in the summer of 2008 and approved by the Pentagon in October. Herb Keinon contributed to this report.