The Israel Air Force (IAF) struck 14 targets early this morning in two waves of airstrikes leading to nine deaths, Palestinian medics said.
The first round of airstrikes targeted nine Hamas bases in Gaza and the second launched a few hours later hit five underground rocket launchers, Israeli security sources said.
The IAF last night also struck a cell in central Gaza which was in the process of carrying out a rocket attack on Israel, killing two Palestinian militants, the Israel Defence Forces (IDF) said.
After nine militants were killed, Hamas vowed revenge on Israeli forces.
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"This is a serious escalation and the enemy will pay the price," Sami Abu Zuhri, the Hamas spokesman in Gaza, posted a warning message overnight on his Facebook page.
"Nine Hamas fighters were killed and four people were wounded in the attacks," the group's armed wing 'Izz ad-Din al-Qassam Brigades' said in a statement.
However, local media reports said two of the nine killed belonged to Islamic Jihad's military wing.
It remained unclear whether the tunnel was used for smuggling or to launch an attack against Israel.
Security sources in Israel said it was possible that the Palestinians who were killed in the tunnel were not hit as a direct result of the IAF attacks.
"The six Hamas terrorists could have been killed from the collapse of a terror tunnel the IAF attacked on Saturday. It is possible that the Hamas operatives sought to fix the tunnel overnight and it collapsed on top of them," they said.
The Sha'ar Hanegev Regional Council area was battered with ten rockets from Gaza with the areas residents instructed by the army to remain in fortified shelters.
Seven rockets hit the Eshkol Regional Council area, one of which started a brush fire, and an additional two rockets landed in open territory in the Ashkelon Coast Council region.