Israel and Palestinian groups are blaming each other for violating a 72-hour truce that broke down just hours after coming into effect on Friday.
An Al Jazeera team arrived at Khuzaa in Khan Younis to find people retrieving decomposing bodies from homes.
Witnesses said six youths had been taken to a house and executed by Israeli forces. The bodies were left there for several days.
One man said: "It is a mass execution of six Palestinian youths who were brought to this bathroom. Look at the gunshots on the wall."