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Fighting through the "spider's web" of tunnels under Gaza could take months, says John Spencer, the chair of Urban Warfare Studies at the Modern War Institute at the US Military Academy in West Point, New York.

He believes there could be more than 500 kilometres (310miles) of tunnels under Gaza in which groups like Hamas are able to live, set up infrastructure and store vehicles and equipment, anywhere up to 50 metres beneath the surface.

"It will be one of the biggest problems Israel and the IDF will face if they launch the ground invasion," he says.

Spencer is a former US soldier and says that while Israel's military has "unique capabilities" in tunnel warfare, clearing tunnels with soldiers is a "nightmare".

(Pic: Members of Al-Quds Brigades, an armed wing of Islamic Jihad Movement, keep guard at tunnels on Gaza-Israeli border Credit: Getty Images)