Israeli forces battling Hamas said they had uncovered an unusually large concrete and iron-girded tunnel, designed to carry carloads of militant fighters from Gaza right up to the border.

Razing or disabling hundreds of kilometres of underground passages and bunkers is among the aims of the offensive Israel launched after Hamas gunmen went on a killing and kidnapping spree in its southern towns and army bases on October 7.

Among sites that Hamas overran in that attack was the Erez border crossing between Gaza and Israel. Just 100 metres (yards) south of the checkpoint, concealed in a sand dune, the military showed reporters the exit point of what it said was a flagship Hamas project.

The tunnel ran down diagonally to a depth of 50 metres, where it expanded to a relatively capacious 3 metres (10 feet) in height and width, with electrical fittings.

Chief military spokesperson Rear Admiral Daniel Hagari put the full length of the tunnel at 4 km (2.5 miles) - enough to reach into northern Gaza City, once the heart of Hamas governance and now a devastated combat zone.

It was "the biggest tunnel we found in Gaza ... meant to target the (Erez) crossing," Hagari said, without specifying whether it was used by Hamas for the Oct. 7 attack.

"Millions of dollars were invested in this tunnel. It took years to build this tunnel ... Vehicles could drive through."

Hamas did not respond to a Reuters request for comment on the Israeli account.

Generally the tunnels shown to the media by the group, or by the Israeli military after their discovery, have been narrow and low - designed for single-file movement of gunmen on foot. The tunnel shown by Hagari had shafts plunging vertically downward that, he said, suggested it was part of a wider network.

The tunnels have been a challenge for Israel's engineers, worried that the networks could conceal hostages held by Hamas. That has slowed an offensive whose steep Palestinian civilian toll has alarmed world powers.

Hagari showed reporters a video of Mohammed Sinwar, brother of Hamas leader Yahya Sinwar and himself a senior operative in the group, sitting in the passenger seat of a vehicle that he said was driving inside the tunnel.

On October 29, Israel's Ynet news site reported that troops killed several gunmen who attacked Erez after accessing the area from a tunnel. Hagari's office did not respond to a query on whether that referred to the tunnel he showed.