There are some 37 million tonnes of debris to clear away in Gaza once the Israeli offensive is over, a senior official with the UN Mine Action Service said Friday.

And unexploded ordnance buried in the rubble would complicate that work, said UNMAS' Pehr Lodhammar, who has run mine programmes in countries such as Iraq.

It was impossible to say how much of the ammunition fired in Gaza remained live, said Lodhammar.

"We know that typically there is a failure rate of at least 10% of land service ammunition," he told journalists in Geneva.

"What we do know is that we estimated 37 million tonnes of debris, which is approximately 300 kilos of debris per square metre," he added.

Starting from a hypothetical number of 100 trucks, that would take 14 years to clear away, he said.

Lodhammar was speaking as UNMAS launched its 2023 annual report Friday.

The war in Gaza between Israel and Hamas erupted when the Palestinian Islamist group Hamas launched an unprecedented attack on Israel on October 7.

The attack resulted in the deaths of 1,170 people in Israel, according to an AFP tally based on official Israeli figures.

Israel vowed to eliminate Hamas, and its ensuing military offensive in Gaza has killed at least 34,356 people, most of them women and children, according to the health ministry in the Hamas-run territory.