At least 27 people have been killed in one of the most violent internal confrontations since Israeli troops withdrew.

Families desperately awaiting the release of the dozen or so Israeli hostages still in Gaza after a first group was freed got an unexpected surprise Monday morning: A number of their loved ones called home while still being held by Hamas.

Medical teams at an Israeli hospital awaiting the arrival of hostages expected to be released from Gaza are "prepared for anything," Noa Eliakim Raz, head of the Returning Hostages Unit at the Rabin Medical Center-Beilinson Hospital in Petah Tikva, told NBC News.

Tensions rose sharply and quickly after two members of Hamas's elite forces were shot dead by gunmen from the powerful Dughmush clan in Gaza City's Sabra neighbourhood. One of them was the son of a senior commander in Hamas's armed wing, Imad Aqel, who now heads the group's military intelligence.

Hamas is reasserting control of parts of Gaza not occupied by Israeli forces as the ceasefire takes hold – amid continuing uncertainty about security in the territory if the group is disarmed.

Hamas released all 20 remaining living hostages held in Gaza on Monday. The bodies of the remaining 28 dead hostages are also expected to be handed over as part of the deal, although the exact

U.S. President Donald Trump suggested Hamas had been given a greenlight for internal security operations it is conducting in the Gaza Strip, saying the group wants "to stop the problems" and "we gave them approval for a period of time".