Gaza rescue teams and medics said Israeli air strikes killed at least 55 people on Thursday, as the military threatened an even larger offensive if hostages were not freed soon.Israel resumed its military assault in the Gaza Strip on March 18, after the collapse of a two-month ceasefire that had brought a temporary halt to fighting in the blockaded Palestinian territory.Israel's army chief, visiting troops in Gaza on Thursday, threatened to expand the offensive in Gaza if hostages were not released.The warning came as the army issued fresh evacuation orders for northern areas of Gaza ahead of a planned attack.Earlier in the day, six members of one family -- a couple and their four children -- were killed when an air strike levelled their home in northern Gaza City, the civil defence agency said in a statement.Nidal al-Sarafiti, a relative, said the strike happened as the family was sleeping.Nine people were killed and several wounded in another strike on a former police station in the Jabalia area of northern Gaza, according to a statement from the Indonesian hospital, where the casualties were taken.In another deadly attack, the bodies of 12 people were recovered after the Hajj Ali family home, also in Jabalia, was struck, the civil defence said.Another 28 people were killed in strikes across the territory, medics and the civil defence agency reported.They came as the Israeli military ordered Palestinians living in the northern areas of Beit Hanoun and Sheikh Zayed to evacuate ahead of an attack.The United Nations has warned that Israel's expanding evacuation orders across Gaza are resulting in the "forcible transfer" of people into ever-shrinking areas.Aid agencies estimate that the vast majority of Gaza's 2.4mn residents have been displaced at least once since the war began.In the aftermath of a strike in Khan Younis, AFP footage showed bodies on the ground, including those of a young woman and a boy in body bags, surrounded by grieving relatives kissing and stroking their faces.Since Israel resumed its military operations, at least 1,978 people have been killed in Gaza, raising the overall death toll to at least 51,355 since the war began, according to the health ministry in the Hamas-run territory.The military acknowledged on Thursday that Israeli tank fire had killed a UN worker in the central Gaza city of Deir el-Balah last month, according to an investigation's initial findings.It had initially denied operating in the area where a Bulgarian employee of United Nations Office for Project Services (UNOPS) was killed on March 19.Bulgaria said it had received an "official apology" from Israel over the killing.The findings came after the military on Sunday reported on a separate probe into the killing of 15 Palestinian emergency workers in Gaza.It admitted that operational failures led to their deaths, and said a field commander would be dismissed.