(Reuters) - It may be only a "dumb" fence, but it´s a big one. Israel hopes it will protect the remote Sinai border from infiltration by enemies exploiting the wandering ways of Bedouin tribes and a perceived surge in lawlessness following Egypt´s political upheavals.

When it is finished in 2013, the 5-metre (16-foot) high barrier of galvanized steel bars and razor mesh -- at this stage minus the smart electronic sensors used elsewhere -- will run most of the 266 km (165 miles) from Eilat on the Red Sea´s Gulf of Aqaba up to the already- closed Gaza Strip on the Mediterranean.

For much of its course, the silvery steel fence weaves up and down among the barren brown hills beside Route 12, a lonely two-lane blacktop through the desert that was closed to traffic after gunmen crossed the border last August and attacked a bus, killing eight Israelis.

On Sunday, the Israel Defense Forces (IDF) announced that the fence had already improved security to the point where Route 12 could now be reopened, although in daylight hours only.

Like no other country in the world Israel is fenced off, to the north with Lebanon and Syria, to the east with Jordan, in the centre by a barrier partly of high concrete walls enclosing the occupied West Bank, and now to the west with Egypt.

"This is a hot border now," said IDF Lt. Col Yoav Tilan at the fence, where welders, pile-drivers and tractors were at work in the empty desert.

On the Lebanon border Israel faces a rocket threat from the Lebanese Shiite Hezbollah movement. On the blockaded Gaza Strip a fortified front line separates Israeli forces from Gaza´s armed Palestinian Islamist group, Hamas. The Jordan River valley is fenced and patrolled along its length.

The landmines, movement detectors and heat sensors that enhance the protective power of Israel´s border fences elsewhere are not yet installed on the Sinai barrier. IDF Bedouin trackers daily inspect a path smoothed in the sand for any sign of nighttime infiltrations.

"This is what we call a dumb fence. It is only one part of our defensive suite," Tilan told reporters on a tour. "It has already been cut once. But they didn´t get through."

SINAI HAS CHANGED

Sinai was relatively quiet for 30 years, but a rapid increase in the flow of migrants from Africa since the mid-2000s highlighted how easy it was to cross the border.

The situation has only become worse after Egypt´s revolution a year ago relaxed the grip of the Cairo authorities on Sinai´s desert tribes.

Israel´s primary concern then and now is that its enemies will exploit any security lapses. Israel says Egypt´s security forces have been paying less attention to Sinai since President Hosni Mubarak was overthrown in February 2011, opening the door to lawlessness that helps terrorist organizations.

Israeli military authorities say Palestinian militants in Gaza, led by Hamas but including Islamic Jihad, are trying to use the peninsula as a back door.

"The fence is part of a security concept intended to stop infiltrations and terror activity in the country," said Brig-Gen Eran Ofir, who heads the 1.3-billion-shekel ($380-million) project. It was first authorized in January 2010 but construction began only in November of that year.

On August 18, 2011, eight Israelis were killed on Route 12 by militants who crossed the unfenced border with automatic weapons and rocket-propelled grenades. Three attackers and five Egyptian soldiers were killed in the ensuing gun battle with the IDF, igniting furious protests outside the Israeli embassy in Cairo.

The IDF says Palestinian gunmen carried out the attack.

"We believe there are other groups with the same plans right now. We could face another terror attack at any time," said an Israeli commander who briefed reporters at the IDF´s regional command post on Route 12. "There is a constantly increasing threat from the Western border, turning into a hostile terror threat."

The IDF says it must treat any criminal activity on the border initially as a terrorist threat. Last Wednesday, it said a border patrol chased off smugglers and found a bomb.

"A smuggling attempt was identified, and the force that operated to stop the smuggling identified a man hurling a suspicious bag and escaping from the scene," the IDF said. "It was discovered the bag contained a powerful explosive device."

This was a reminder that smuggling routes over the border "are constantly being used by terror organizations", it said.

There have been no lethal attacks since August, however.

The commander, who asked not to be named, said cooperation with Egypt is good. Liaison officers talk daily and commanders meet in person every two weeks or so. But Israel hopes that as Egypt is stabilized, it will put more into policing the desert.

Israel says while it has increased manpower on the border, Egypt´s force is two battalions below permitted strength.

"The quality of intelligence is very low," the officer said.

BORDER MOSTLY EMPTY

Some 55,000 migrants have entered Israel from Sinai since 2006, and the flow is accelerating. In all of 2006 there were 2,777. In the past quarter they averaged about 2,500 per month.

People-smugglers equipped with 4x4s and night-vision scopes deliver them to the rocky ridges north of Eilat. They make no effort to escape once they reach Israel but surrender to the IDF, hoping to be processed as asylum seekers.

The Israeli commander said 90 percent of Africans infiltrating Israel are economic migrants seeking a better life. Many of the migrants are educated city dwellers with skills and even professional qualifications, he said.

Their journeys are well organized by companies specializing in the trade and typically begin with a flight from Eritrea to Cairo, from where they cross the Suez Canal by road into the Sinai peninsula, completing the final trip to the border over desert tracks at night. It takes about two and a half days.

"The Bedouin smugglers are excellent drivers by day or night. They can outrun us in most places and they know the terrain very well," the commander said.

Thousands of migrants in legal limbo gravitate to the gray economy of the interior. Israel recently adopted stiff penalties to deter the influx, permitting detention for up to three years, to make the point that the country is not a soft route to Europe.

Ahmed Youssuf, 23, is one migrant who got in under the wire, arriving two months ago from Sudan, he says. He already has a job -- as a welder on the fence. (Editing by Sonya Hepinstall) (© Thomson Reuters 2012. 02/27/12)