Kissufim (Israel) (AFP) - The Israeli army on Thursday revealed details of a massive underground barrier being built along the border with the Gaza Strip in a bid to neutralise the threat of Palestinian attack tunnels.

Eventually stretching some 65 kilometres (41 miles), the concrete wall will be accompanied by motion sensors designed to detect tunnel digging and is expected to be completed by mid-2019.

The project had been previously announced, but details of its construction had been kept secret until Thursday, when journalists were allowed to view aspects of it.

The details were unveiled days after the army destroyed what it described as a tunnel intended for attacks stretching from the blockaded Palestinian enclave into Israel and eventually Egypt, at least the third uncovered and demolished in less than three months.

Tunnels were among Hamas's most effective tools during the 2014 war with Israel, with militants using them to enter the Jewish state, carry out attacks and at times even return to Gaza through the underground passages.

The devastating 2014 conflict killed 2,251 Palestinians, while more than 10,000 were wounded and 100,000 were left homeless.

On the Israeli side, 74 people were killed, all but six of them soldiers.

Hamas and other militant groups in Gaza say tunnels are needed for defence.

An Israeli state inquiry published last year accused Prime Minister Benjamin Netanyahu and top army brass of being unprepared for the "strategic threat" of the tunnels from the enclave run by Islamist movement Hamas.

But with the new underground barrier and sensors detecting movement, militant groups would no longer be able to build and use tunnels, a senior Israeli army official said.

"They understand that the strategic weapon of underground tunnels crossing the border is going to end," the official told reporters.

- 'Deep enough' -

Workers -- local and from abroad -- have been labouring around the clock for nearly a year.

The barrier is being built on Israeli territory, east of the existing border fence, with four kilometres completed so far -- in the area of the town of Sderot, off the northern Gaza Strip, and the Nahal Oz area near Gaza City.

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The technique used is similar to that for building support walls for high-rise buildings or underground parking lots, the military official said at one of the barrier construction sites along the border.

Heavy machines dig a deep, narrow trench, filling it with bentonite slurry that keeps the trench from collapsing.

A metal reinforcing cage is inserted, with tubes sucking out the slurry and then filling the trench with cement drying into a wall approximately a metre wide.

Attack tunnels from Gaza can reach the depth of dozens of metres, with the Israeli army official only saying the new barrier would be "deep enough".

A new, eight-metre high border fence being erected atop the underground wall will further prevent infiltrations of Gazans into Israel, the senior official said.

Speaking to reporters near the Israeli community of Kissufim, where an attack tunnel built by Islamic Jihad was demolished in late October, Israeli military spokesman Lieutenant Colonel Jonathan Conricus said the underground wall would be the first such "complete underground barrier."

The operation to demolish the tunnel in October left 12 militants dead.

"Any attempt to tunnel into Israel ... will be detected and targeted" by the army, Conricus said.

But eliminating the tunnel threat would not mean that Gaza militants would cease their attacks on Israel, the senior military official said.

"They're training, building forces for the sea and land," he said.