Among those killed were senior commanders of terror groups while another 11 were wounded in the explosions.
“The latest Zionist crime is a grave escalation against our people and against our resistance aimed at harming our steadfastness and unity," a statement issued by Hamas said.
“It is part of widening efforts to reverse Palestinian unity and maintain division as it was,” it added.
One of those killed was head of the PIJ's armed wing in central Gaza, Arafat Abu Marshould and his deputy Hassan Abu Hasnin.
A source in Gaza informed Ynet that two of the people killed belonged to the military arm of the pro-Iranian Palestinian Islamic Jihad
It later emerged that a Hamas militant identified as Marwan al-Ara was also killed in the explosion, along with Misbah Shubir, who may have been the Hamas field commander.
Unlike a statement issued by the PIJ, which said Israel’s actions were a “declaration of war” and added that it would examine all options for reprisals, Hamas’s remarks were devoid of any mention of a possible retaliation.
“The resistance to the occupation in all its forms and the various ways in which it is strengthening is a natural right that is guaranteed for our people,” the Hamas statement went on.
“The continued escalation of the Zionist enemy and the execution of its crimes will only increase our adherence to the way of unity and increase our reliance on the weapon of resistance accordingly in our attempt to settle the score with (Israel)."
Earlier, PIJ warned they would be assessing the nature of their response in the coming hours. Anticipating possible rocket attacks, the IDF positioned Iron Dome batteries near the Gaza border.
A spokesperson for the PIJ, Da’ud Shihab, issued a warning that fell just shy of taking explicit responsibility.
“This tunnel is part of our deterrence policies to defend the Palestinian people,” Shihab said. “We won't hesitate to defend our land. The Israeli attack that harmed a group of fighters and civilians is a dangerously aggressive escalation, and a new attempt to shuffle the cards. We are examining all possibilities in order not to lose the option of responding to this aggression.”
The Fatah movement also reacted to Israel's explosion of the terror tunnel, condemning it as "the Israeli crime that harmed the Palestinian people in the Gaza Strip."
A statement issued by the movement said that "this Israeli crime is part of the policy … that successive Israeli governments have taken in an attempt to evade the trials of corruption they face at the expense of the blood of our people."
The movement also stressed its firm position to continue in completing their unity talks with Hamas, despite Israel's attempt to sabotage it.
"Israel is trying to escalate the situation in order to keep us divided," says Fatah spokesman Osama Al-Qawasmi, echoing the sentiments voiced by Hamas.
By later Monday evening, Israel issued its own statement, saying that the killings that accompanied the tunnel's destruction were inintentional.
"There was no intention of eliminating senior militants," IDF Spokesperson, Brigadier General Ronen Manelis said.
According to Manelis, Islamic Jihad officials ran into the tunnel to head the rescue force, who accounted for the majority of those killed.
"Our action was defensive, in our territory, and if there is a response, it will be answered as we know how to respond in these situations," he added.
The IDF reported that the tunnel did not have an exit point in Israel and was not intended for any immediate attack.
Roi Kais, Elior Levy, Matan Tzuri and Yoav Zitun contributed to this report.