Maj. Elim Saad, right, deputy commander of an Israeli battalion stationed at the Israel-Sinai border, watches from a post at Nitzana overlooking Route 10, because of the perceived threat from ISIS-linked Sinai rebels. (Ruth Eglash/The Washington Post)

Below a dusty hilltop near this southern Israeli village, a neat, two-lane highway snakes into the distance across the rugged desert landscape.

The most striking feature of Route 10, as the road is designated, is the absence of cars. Under a military security advisory, its entire 113-mile length is almost permanently off-limits to civilian traffic — the only stretch of highway in tumultuous Israel thus restricted.

The reason: It runs along Israel’s border with Egypt’s Sinai Peninsula, which for the past five years has been the scene of a violent insurgency mounted by Bedouin clans against the Egyptian government. In 2014, the Sinai rebels, known as Wilayat Sinai, declared their allegiance to the Islamic State and since then have increasingly embraced the group’s extremist ideology.

Cross-border attacks by the militants and spillovers from their battles with the Egyptian army remain sporadic, but deadly, and Israeli authorities say the threat is growing.

The most significant incident occurred in 2011, when eight Israelis were killed in a multipronged attack, including on a passenger bus on the border highway, by militants alleged to have crossed into Israel from Sinai. Israeli soldiers pursuing fleeing attackers entered Egyptian territory, where they opened fire — inadvertently, according to Israel — on Egyptian soldiers, killing at least five. Since then, Route 10, which runs from a point near the Gaza Strip to one not far north of Eilat, Israel’s southernmost city, has been opened to civilians only on rare occasions.

Last month, during the Jewish high holy days, the army allowed Israelis celebrating the holiday to access the scenic road and visit its renowned nature spots.

“The road is on Israeli land, and that is why we allow people to drive on it from time to time,” said Maj. Elim Saad, deputy commander of the Karakal Battalion, which is stationed along the Israel-Sinai border. “We have an army, and it’s there to protect our civilians.”

But the army lacks the resources to keep the road open permanently, Saad said, noting that soldiers had to set up and staff lookout posts along the route for the recent operation. The road was closed as soon as the holiday ended.

A few days later, a 15-year-old Israeli was killed by gunfire from the Egyptian side. The youth had been helping his father, a Defense Ministry employee, carry out maintenance on the new, high-tech fence that runs along the border next to the road. While both Israel and Egypt say the shots were fired from Egypt, the two sides are still investigating the incident.

The Egyptian military’s resources in Sinai are stretched, too, as it battles to contain the Bedouin-led insurgency there. The challenge intensified after the tribes aligned themselves with the Islamic State and received money, training and other resources in exchange.

A view from Israel’s Negev Desert shows the fence Israel has constructed along its border with Egypt's Sinai Peninsula, adjacent to Route 10. (Amir Cohe/Reuters)

Although Wilayat Sinai is focused on its fight with the Egyptian military, it has also spoken in the past about attacking Israel, according to Zack Gold, a non­resident fellow with the Atlantic Council’s Hariri Center in Washington. In addition, the Egyptians and the Israelis allege that it maintains a tactical alliance with the Palestinian militant group Hamas, which rules the Gaza Strip and is Israel’s sworn enemy. Although Hamas has denied that, many experts say the Sinai rebels support Hamas, helping it acquire and smuggle in arms.

“No one wants another conflict,” Gold said, referring to the tension between Israel and Hamas that has produced three wars in the past decade. But, he added, “they say it is just around the corner, and eventually it will happen.”

A senior Israeli army official agreed that the Sinai militants do not represent an immediate threat to Israel.

“Their main goal is to get rid of the Egyptians and create an Islamic caliphate in Sinai,” he said, speaking on the condition of anonymity in keeping with military protocol.

But he also noted the Islamic State’s greatly enhanced ability to carry out attacks in Sinai and the possibility that it could suddenly turn its attention to Israel.

Meanwhile, for Egypt, the battle in Sinai has never been deadlier, with near-weekly reports of armed forces personnel killed in fighting in the region, including four in late October.

Although the Egyptians also claim to have killed hundreds of militants in Sinai, the military’s accounts of the battles there — including details such as the destruction of three warehouses used to store bombs — reveal the Islamic State’s potent capabilities. The reports have been impossible to verify independently, however, because the government has banned journalists from the area.

Egypt’s Western allies, especially the United States, have voiced concern about the growing Islamic State presence in Sinai. Senior U.S. military and political officials, including Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, have traveled to Cairo this year to discuss security there and in the broader region.

Egypt has worked increasingly closely with Israel to defuse the Islamic State threat, according to officials from both countries.

The Israeli military official said that Israel and Egypt have shared interests now that they are fighting a mutual enemy and that they also respect each other’s sovereignty. That is one reason Israel prefers to keep Route 10 closed, even though more dangerous roads — those running along the border with Syria and Lebanon and through the West Bank, for example — remain open.

“When it comes to Syria or Lebanon, Israel knows how to respond and will not hesitate to attack forces­ across the border,” Gold said. “But with Egypt, the modus operandi for responding is more limited. They cannot just fire back when shots are fired because it could end up resulting in the death of an allied soldier, and that would embarrass their Egyptian partner.”

Sudarsan Raghavan in Cairo contributed to this report.