An investigation has found that Israel's military was unprepared for the threat posed by tunnels dug by Palestinian militants in the Gaza Strip in the 2014 war between them, reports said Tuesday.

The findings are likely to pressure Prime Minister Benjamin Netanyahu, who has faced criticism from political rivals over allegations that not enough was done to prepare for the tunnel threat.

Militants in Gaza, run by Islamist movement Hamas, used the tunnels during combat in the 2014 war, including to carry out cross-border attacks.

Tuesday's reports cited leaked findings from an internal military investigation, which was not released publicly.

Israel's army radio said the probe found that military brigades were not prepared for the magnitude of the threat the tunnels posed.

It also said that the "enemy's command and its capacity to fire rockets remained intact continuously until the end of the operation and the number of Hamas victims was lower than would have been expected given the firepower used".

Israel's army said in a statement late Tuesday: "The conclusions of the investigation have been implemented."

"The IDF recognises underground warfare as a major challenge in modern warfare, and has placed it as its top priority," it said, referring to the Israeli military.

Despite the findings, the Gaza Strip suffered an overwhelmingly higher number of victims and amount of damage in the 2014 war.

The 50-day conflict killed 2,251 Palestinians and left 100,000 homeless.

On the Israeli side, 73 people were killed, most of them soldiers.

A UN report said both Israel and Palestinian militants may have committed war crimes, decrying "unprecedented" devastation and human suffering.

Israel's preparations ahead of the Gaza war have faced criticism before.

In May, Israeli media reports said a draft of a separate inquiry by the state comptroller found that "the security establishment did not have a comprehensive plan for dealing with Hamas's offensive tunnels".

Netanyahu has defended preparations ahead of the conflict, saying his cabinet repeatedly discussed the tunnel threat beforehand.