caused by rain and flooding, a security source said.
The tunnel collapsed overnight in the area of Jabalia in the north of the Palestinian enclave after several days of rainfall, the security source in the area said on condition of anonymity.
"The resistance tunnel collapsed last night due to the weather and flooding," the source said, adding that the tunnel belonged to Hamas, the Islamist movement that rules the Gaza Strip.
"There were 11 resistance men inside. Three of them escaped in the first hour after the accident, but the security operation... continues to search for the eight others."
Hamas's armed wing, Ezzedine Al-Qassam Brigades, later confirmed in a statement that one of its tunnels collapsed.
It said searches were continuing for the missing, without providing a number or further details.
It comes at a time of renewed focus on tunnels in Gaza, with Israel accusing Hamas of reconstructing them following its 2014 war with Palestinian militants in the enclave when many were destroyed.
Israel says the tunnels could be used to carry out fresh attacks against it.
Such collapses have previously occurred in the coastal strip, which is under an Israeli blockade and has seen three wars with the Jewish state since 2008.
On Saturday, a tunnel collapse killed a 30-year-old man, according to Hamas officials. Residents said the tunnel was located in Al-Maghazi in the central Gaza Strip.
In December, 14 Palestinians were rescued after being stranded for hours in a tunnel near the Egyptian border when it flooded and partially collapsed.
At the end of 2014, Egypt began the construction of a buffer zone in the northern Sinai Peninsula, on the border with Gaza, including destroying hundreds of tunnels it says are used for smuggling weapons.Israel's blockade severely restricts the movement of people and goods into and out of the enclave.
Egypt's sole border with Gaza has also remained largely closed following the 2013 overthrow of the country's president Mohamed Morsi of the Muslim Brotherhood, effectively trapping the 1.8 million Gazans into the territory.