Israeli forces have found 800 shafts leading to Hamas' vast subterranean network of tunnels and bunkers since a Gaza ground operation began on Oct 27, and have destroyed more than half of them, the military said on Sunday.

The Palestinian Islamist group said before the now eight-week-old war in the Gaza Strip that it had hundreds of kilometres of tunnels - a network comparable in size to the New York subway system - to protect and serve as operational bases.

That has made them prime targets for Israeli air strikes with penetrating munitions and army engineers using mapping robots and exploding gel that can be poured into the passages.

"The tunnel shafts were located in civilian areas, many of which were near or inside civilian buildings and structures, such as schools, kindergartens, mosques and playgrounds," the military said in a statement on Sunday.

The statement, summarising anti-tunnel operations so far, followed near-daily accounts to the media by troops who said they uncovered access shafts in civilian sites.

The war's civilian toll has increasingly worried world powers. Washington urged Israel to use caution on Saturday.

Of some 800 shafts discovered, the military said, 500 had been destroyed using a variety of operational methods, including by "detonation and by sealing off". It added that "many miles" of main tunnel routes had also been destroyed.