Heavy machinery is seen on the Israeli side of the border with Gaza Thursday, Sept. 8, 2016.

Defense officials say Israel has begun work on an underground barrier along the border with Gaza meant to block Hamas militants from tunneling into Israel.

The officials say the concrete barrier is set to run dozens of meters (hundreds of feet) deep and will ultimately stretch along the entire border with Gaza. Israel is currently building an initial phase of the barrier over a small stretch of land measuring dozens of meters.

Work crews were busy along the Israel-Gaza border Thursday using cranes and heavy machinery. Caravans are scattered around the site, while large metal pipes run along the ground.

The officials, speaking on condition of anonymity because they were discussing a classified matter, say the barrier’s construction could take years. Hamas militants have used underground tunnels to attack Israelis.

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