Palestinian Islamist movement Hamas has denied that it or any other militant group built a tunnel under two UN schools in Gaza after its discovery drew a strong UN protest.

Over the years, Gaza's Hamas rulers have built a labyrinth of tunnels, some passing under the border into Israel which they used to launch attacks during their last conflict in 2014.

On June 1, the UN agency for Palestinian refugees (UNRWA) found "part of a tunnel that passes under two adjacent agency schools in the Maghazi camp" during construction work, spokesman Christopher Gunness said on Friday.

Hamas late Friday "strongly condemned" the UNRWA statement, saying it would be exploited by Israel to "justify its crimes".

Hamas had clarified the issue "with all factions and resistance forces, who clearly stated they had no actions related to the resistance in the said location," the movement said, adding that it respected UNRWA's work.

Gunness said that the tunnel "has no entry or exit points on the premises nor is it connected to the schools or other buildings in any way".

"UNRWA condemns the existence of such tunnels in the strongest possible terms. It is unacceptable that students and staff are placed at risk in such a way," he said.

Gunness said the agency had "robustly intervened and protested to Hamas in Gaza".

He said UNRWA will seal the tunnel, which was discovered while the schools were empty during the summer holiday.

Attack tunnels were a key weapon for Hamas during the 2014 Gaza war.

Hamas also built a vast network of tunnels under Gaza's border with Egypt to smuggle goods and allegedly weapons.

The Israeli army found and destroyed several tunnels during the 2014 war.

Israel and Hamas have fought three wars in the Gaza Strip since the group wrested control of the territory from the rival Fatah movement of Palestinian president Mahmud Abbas in 2007.

Source: AFP