Despite Egypt’s relentless efforts to seal off its side of the Hamas-run Gaza Strip, a steady if light stream of Gazans are managing to leave the Strip and join the ranks of Islamic State fighters locked in combat with Egyptian forces in the Sinai, sources in the Strip said Thursday.

The Gaza fighters are crossing mainly via the last few tunnels still connecting the Sinai Peninsula to Gaza. Egypt has closed most of the smuggling tunnels in recent years.

Some of those crossing to join IS have fought against Israel in the past, either as members of Hamas or of Salafi-jihadist terror groups, the sources said.

The fighters are crossing into Egypt with the knowledge of Hamas’s military wing, which supervises the tunnels, the sources said. The Egyptian administration is aware of the trend and trying to prevent it.

The fighters are leaving in small groups, but are gradually coming to constitute a fairly substantial number of recruits to Sinai Province, the IS affiliate that is fighting the Egyptian army in Sinai. The sources did not have a firm figure for how many fighters have crossed the border.

Two of the best-known operatives who have crossed into to Egypt are Mahmoud Za’arah and Muhammed Shahin, both of whom have played an active part in clashes with the IDF. Za’arah and Shahin are said to be among those training Sinai Province fighters.

According to the sources, there are currently just 10-15 active tunnels between Gaza and Egypt, a drastic decrease compared to three years ago, when the number was 650-700. Only a few solitary tunnels connect the Rafah crossing area and the triangle of borders — Egypt, Israel and Gaza — in the Kerem Shalom area, they said.

Egypt has flooded tunnel areas and demolished hundreds of houses close to the border at and around Rafah. Sinai Province gunmen attack Egyptian military targets on a near-daily basis.

The fact that Gaza gunmen are crossing into Egypt to fight with IS further underlines the scale of Egypt-Hamas hostility. Cairo is said to be steadfastly opposed to any idea of easing the pressure on Hamas by giving Gaza access of some kind to a seaport. The Hebrew-language Haaretz daily reported on Wednesday that some senior IDF officials support the idea, and that a range of possible port arrangements — including an artificial offshore island to hold a port — are being discussed at senior military and political levels, in part because of the worsening economic situation in Gaza and consequent Israeli concern over renewed conflict.

Prime Minister Benjamin Netanyahu and Defense Minister Moshe Ya’alon flatly oppose all such ideas, however, the newspaper report said.