Seven of the slain Palestinians are members of the al-Qassam Brigades of the Hamas movement.
The Brigades stated the six fighters have been killed when the Israeli army fired missiles into a siege-busting tunnel in Rafah, in the southern part of the Gaza Strip.
Their bodies were found on Monday at dawn.
The fighters have been identified as Ibrahim al-Bal'aawy, 24, Abdul-Rahman az-Zamely, 22, Mustafa Abu Morr, 22, his twin brother Khaled, Yousef Sharaf Ghannam, 22, and Jom'a Abu Shallouf, 24.
A Qassam fighter, identified as Ibrahim 'Abdeen, died of wounds suffered in an earlier shelling in Rafah.
Sami Abu Zohri, spokesperson of the Hamas movement, stated the assassination of the fighters is a serious escalation, adding that "the enemy will pay a heavy price".
Dr. Ashraf al-Qodra, spokesperson of the Ministry Of Health in Gaza, stated that five Palestinians, including a child and two young girls, have been nursed when an Israeli missile detonated near their homes, in Beit Hanoun, in northern Gaza.
Another airstrike targeted al-Qarara town, east of Khan Younis in the southern part of the Gaza Strip.
Missiles have also been fired into agricultural lands in Shekh Radwan in Gaza, causing damage but no injuries.
Two fighters, identified as Mazen al-Jedya and Marwan Salim, have been killed by Israeli missiles in al-Boreij refugee camp. Their bodies have been severely mutilated.
Palestinian resistance groups in Gaza claimed responsibility for firing dozens of shells into adjacent Israeli areas in retaliation to the Israeli military escalation.