Hamas has killed 18 Palestinians suspected of collaborating with Israel, a day after Israel killed three of the group's top military commanders in an airstrike on a house in southern Gaza Strip, witnesses and a Hamas website said.
A Gaza security official said the first batch involved 11 people who were killed early on Friday at the Gaza City police headquarters.
Six more were killed later in the day in a public execution in a central Gaza square, according to a Hamas website and witnesses cited by Reuters news agency.
Three suspected collaborators were also killed on Thursday.
The victims, their heads covered and hands tied, were shot dead by masked gunmen dressed in black in front of a crowd of worshippers outside a mosque after prayers, witnesses and al-Majd, a pro-Hamas website, said.
The Gaza security official said the 11 men had previously been sentenced by Gaza courts, reported the Associated Press news agency.
He spoke on condition of anonymity because he was not authorised to discuss the incident with reporters.
Al Jazeera's Jacky Rowland, reporting from West Jerusalem, said Israel's intelligence services rely, in part, on informers to pinpoint the whereabouts of Hamas leaders.
"Israel has a long and successful history of recruiting collaborators and informers both in the West Bank and in the Gaza Strip, and they do so through a variety of different means: sometimes it is financial inducements; other times it is blackmail, bullying, threats, promises [and] maybe intimidating family members," Rowland said.
She said that by publicising the retribution brought down on the 18 people, Hamas was sending a deterrent to other Palestinians.
The announcement came as five Palestinians were killed by an Israeli air strike, the latest since Egyptian-led ceasefire talks collapsed three days ago.
Ashraf al-Kidra, a Gaza health official, said on Friday some of the victims were workers at a livestock farm that was hit in the airstrike.
The Israeli military said it carried out 20 airstrikes early on Friday, targeting rocket launchers and weapons sites. It said a mortar strike from Hamas critically injured one child in the Negev region area.
Friday's fighting came as funerals were held for three senior Hamas commanders killed by Israeli air raids.
Al-Qassam Brigades, Hamas' military wing, said Mohamed Abo Shamaleh, Raed al-Attar and Mohamed Barhoum were killed in an attack in Rafah on Thursday, little more than a day after an attempt on the life of its leader Mohammed Deif.
The killed commanders' supporters later took over the streets as their funeral procession snaked through Rafah, which bears scars of Israeli bombing from previous days.
Another 31 people were killed in other Israeli strikes in Gaza since on Thursday, raising the overall death toll to 2,087 in 46 days of conflict.
Israel meanwhile said it was rotating 10,000 troops - meaning fresh soldiers were being prepared for possible future operations - a day after the Israeli prime minister, Benjamin Netanyahu, said Israel's offensive may be an extended operation.
Hamas condemned the assasinations, with Sami Abu Zuhri, the group's spokesman, calling them a "big Israeli crime" for which it would pay.
The fighting resumed three days ago after Egyptian attempts to broker an end to the monthlong war, with Palestinians firing dozens of rockets and Israel responding with airstrikes across Gaza.