Hamas's deputy leader has condemned the "unlawful" killing of six Palestinians in the Gaza Strip who were suspected of giving information to Israel.

Moussa Abu Marzouk, in comments posted on his Facebook page urged the Islamist Hamas administration ruling Gaza to ensure that no one takes the law into their own hands.

"Punishing collaborators and especially those involved in the killing of our leaders must only be carried out in accordance with the law and through the legal procedures," Abu Marzouk said.

Citing an unidentified security source, Hamas's al-Aqsa radio station said the six men were shot dead on Tuesday after being "caught red-handed" working for Israel, which has battered Gaza with air strikes for the past eight days in an offensive it says is meant to stop rocket fire from the Palestinian enclave.

"They possessed hi-tech equipment and filming equipment to take footage of positions," al-Aqsa radio station said.

Gunmen chained the body of one of the alleged collaborators to a motorcycle and dragged it throughout the main streets of Gaza City. At least one body was beaten, kicked and trampled.

Troubled by graphic photographs and footage of the incident, Abu Marzouk added: "The way those collaborators were killed and the images after their death were completely unacceptable."

"Those who did it should be punished and it must not be repeated," said the Hamas official, who is currently in Cairo.