Hamas’ military wing announced on social media on Thursday that seven of its fighters were killed when a tunnel collapsed in the northern Gaza Strip on Tuesday.

“Al-Qassam Brigades mourn the death of seven Qassam members who were (killed) during their work inside resistance tunnels in Gaza,” the Izz al-Din al-Qassam Brigades posted on their Twitter account.

The group identified those who died in the tunnel as Thabet al-Rifi, Ghazwan al-Shubaki, Izz al-Din Qassem, Wassim Hassouneh, Mahmoud Basal, Nidal Odeh, and Jaafar Hamadeh.

Hamas security sources had previously said in a statement that more than 10 fighters were in a tunnel when it collapsed due to heavy rain on Tuesday. The Qassam Brigades said the fighters had been working on rebuilding the tunnel at the time of the accident. The fate of the remaining fighters had yet to be specified.

Earlier in January, an al-Qassam member was killed by an electric shock in a tunnel in the southern Gaza Strip.

Gaza’s tunnel networks are notoriously dangerous. The Institute for Palestine Studies reported in 2012 that Hamas authorities had counted 160 deaths inside the tunnels since the Israeli blockade began in 2007, and in August 2014, al-Jazeera reported that figure to be as high as 400.

While the tunnels are used by Hamas as a source of tax revenue and inflow of weapons, they also supply highly-demanded necessities for Gaza’s 1.8 million residents under the blockade, including food, medicine, as well as infrastructure materials like concrete and fuel.

The movement has reportedly expanded the tunnel network — used mainly for military purposes in the northern Gaza Strip and smuggling in the south — since Israel’s 2014 offensive on the besieged enclave left much of it destroyed.

Senior Hamas official Ismail Haniyeh Israel last month said: “Resistance is now stronger than it was during the last war,” warning that Israeli forces would face a “heavy toll” if they carried out another war, without specifying efforts in tunnel expansion.

(MAAN)