(CNN)Eight people were killed when the Israeli military struck a tunnel leading into Israel from Gaza, according to the Palestinian Ministry of Health in Gaza.
The Israeli Army said earlier it had "neutralized a terror tunnel" leading into southern Israel from Khan Younis in the south of Gaza.
Israeli Army spokesman Lt. Col. Jonathan Conricus said the tunnel, which he described as kilometers away from the nearest Israeli village of Kissufim, had been detonated on the Israeli side of the Israel-Gaza border, close to the security fence that separates the two territories.
Asked about reports that senior members of the militant Islamic Jihad group had been killed in the operation, an IDF spokesman said there had been "no intention in any way or at any stage" to target senior figures.
The Palestinian Ministry of Health said the number of fatalities could rise significantly because a large number of people are still missing after the tunnel's collapse.
The strike comes just two days before internal control of Gaza's border crossings is due to pass from Hamas to the Palestinian Authority, as part of a reconciliation agreement reached between Hamas and Fatah, the party of Palestinian President Mahmoud Abbas.
Hamas, though not claiming the tunnel as one of theirs, said in a statement that the Israeli action was aimed at undermining the reconciliation agreement; in an earlier posting on its Twitter account, Hamas had called it a dangerous escalation that threatened a "new war against Gaza people."
An Islamic Jihad leader, Mohammad Al Hindi, said in a statement: "The truce with the enemy is over and the response will be the size of the crime."
In a statement carried on the official Palestinian news agency Wafa, Fatah condemned what it called "the Israeli crime," saying "the blood [of those killed] would not be wasted." It said it would move ahead with the unity agreement with Hamas.
Earlier, the IDF's Conricus said Israel held Hamas accountable for "all hostile activity" emerging from Gaza, though he stopped short of directly accusing Hamas of being responsible for the tunnel. He said Israel did not intend to escalate the situation but stood prepared for all eventualities.
He would not say how the tunnel was detonated, but described the technology used to detect it as "advanced and groundbreaking."
Later, an IDF spokesperson rejected Hamas and Palestinian Ministry of Health claims that Israel had used poison gas in its operation against the tunnel, telling CNN the IDF had only used conventional ammunition in the action.
The spokesperson also said the IDF had told local Israeli farmers not to work near the security fence; however, the spokesperson said reports that people living in towns and communities near Gaza had been told to avoid large gatherings outdoors were false.
During the 2014 Gaza war, Hamas militants launched surprise attacks from tunnels that crossed under Israel's security fence and into Israel. Identifying and destroying the tunnels became a major goal of the war for the Israeli military, which found many tunnels that were more than a mile long and 60 feet deep.
Two days ago, the United Nations Relief and Works Agency for Palestine Refugees revealed it had recently discovered the existence of what appeared to be a tunnel underneath one of its schools in Gaza. It said it had rendered the school safe and sealed the cavity created by the tunnel. The agency said it had protested "the violation of the sanctity and disrespect of the neutrality" of UN premises to what it called the "relevant parties" responsible for the tunnel.