The Defense Ministry has begun the final phase of construction of a 20-foot high galvanized steel fence that will completely surround the Gaza Strip, Israeli officials said Sunday.

The barrier will extend 65 kilometers (40 miles) miles around the enclave and sit atop the subterranean concrete wall Israel is constructing around the Gaza Strip to block terrorist groups’ attack tunnels from the coastal enclave.

Prime Minister Benjamin Netanyahu said the barriers were needed to “prevent the infiltration of terrorists into our territory,” at the start of weekly cabinet meeting.

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The fence will connect to the barrier recently built out into the Mediterranean Sea from north of Gaza, the Defense Ministry said in a statement.

The overall Gaza barrier project is due to be completed by the end of 2019, according to the army.

“On Thursday, we began work on the final component of the Gaza Strip border barrier project. The obstacle is unique and specially designed to protect against the threats from the Strip and to give a superior solution to preventing infiltration into Israeli territory,” said the head of the project, Brig. Gen. (res.) Eran Ofir.

The barrier project is expected to cost approximately NIS 3 billion ($833 million), with each kilometer of the underground portion of the barrier costing approximately NIS 41.5 million ($11.5 million). The above-ground fence is significantly cheaper, at just NIS 1.5 million ($416,000) per kilometer.

The new fence surrounding the Gaza Strip will be constructed within Israeli territory, a few dozen meters east of the current shorter, more easily penetrable fencing. The old barrier will not be removed.

According to the Defense Ministry, the new galvanized steel fence will weigh approximately 20,000 tons and comes equipped with a number of sensors and other “modern security components.”

The barrier is being constructed jointly by the Israel Defense Forces-Defense Ministry Borders and Security Fence Directorate, run by Ofir, who has overseen the construction of barriers along Israel’s borders with Egypt, Jordan, Syria and Lebanon.

In 2016, Israel began construction of the new barrier around the Strip, focusing first on the underground portion, following the 2014 Gaza war in which Hamas used subterranean attack tunnels to deadly effect against Israeli troops.

Over the past two years, work has persisted on the underground sensor-studded concrete wall, despite regular riots and clashes along the border and occasional attacks on the construction sites.

In addition, the Defense Ministry built a barrier extending out from Israel’s coast aimed at preventing maritime infiltration from Gaza, as occurred in the 2014 war when a team of Hamas naval commandos landed on the beach near the community of Kibbutz Zikim before they were killed by Israeli forces. Construction of the undersea wall and breakwater was completed last month.

The new above-ground fence will begin at the Egyptian-Israeli-Gaza border, near Kerem Shalom, and will continue out to the sea barrier, according to the Defense Ministry.

“The above-ground barrier… is another important element in the defense of the [Israeli] communities surrounding Gaza, which already includes: the sea barrier, which provides a response to terrorist infiltration from the sea to the west, and the underground barrier that surrounds the Strip and is meant to prevent the digging of terror tunnels into Israel,” the ministry said.

The military proposed building the barrier following the 2014 Gaza war, known in Israel as Operation Protective Edge. During the fighting, Hamas made extensive use of its tunnel networks to send fighters into Israel as well as to move its terrorist operatives and munitions within the Gaza Strip.

Hundreds of people, some Israeli and others from abroad, are involved in the project, wearing flak jackets and under guard by IDF soldiers as protection against attack from terror groups in the Strip.

Concrete factories were built next to the Gaza Strip to speed up construction.

To build the underground wall, the workers use a hydromill, a powerful piece of drilling equipment that cuts deep, narrow trenches into the earth, which was brought to Israel from Germany.

In addition to opening up the ground where the barrier will be constructed, the hydromill also exposes any previously undiscovered or newly dug Hamas tunnels that enter Israeli territory. The space left behind by the hydromill — and any Hamas tunnels that get in the way — is then filled with a substance known as bentonite, a type of absorbent clay that expands when it touches water.

This is meant to prevent the trenches from collapsing, but also has the additional benefit of indicating the presence of a tunnel, as the bentonite would quickly drain into it. Workers then pour regular concrete into the trench. Metal cages with sensors attached are then lowered into the concrete for additional support.