In Pics | Here's how Hamas secretly built a 'mini-army' to fight Israel

'Necessity is the mother of invention,' said Ali Baraka, a senior Hamas official, adding that the group had long drawn on money and training from Iran and Iranian regional proxies like Lebanon's Hezbollah, while bolstering its own forces in Gaza, according to Reuters news agency.

Difficulties in importing weapons meant that over the past nine years "we developed our capabilities and are able to manufacture locally", said Baraka, who is based in Lebanon.

In the 2008 Gaza war, Hamas rockets had a maximum range of 40 km (25 miles), but that had risen to 230 km by the 2021 conflict, Ali Baraka, a senior Hamas official, added.

Today the secretive and sprawling organisation is unrecognizable from the small Palestinian group that issued its first leaflet 36 years ago protesting at Israeli occupation, according to Reuters interviews with 11 people familiar with the group's capabilities, including Hamas figures, regional security officials and military experts.

"They are a mini-army," a source close to Hamas in the Gaza Strip told Reuters news agency as he declined to be named due to the sensitivity of the matter. He said the group had a military academy training a range of specialisations including cyber security, and boasts a naval commando unit among its 40,000-strong military wing.

By contrast, in the 1990s Hamas had less than 10,000 fighters, according to the globalsecurity.org website.

Since the early 2000s the group has built a tunnel network under Gaza to help fighters melt away, house weapons factories and bring in weapons from abroad, according to a regional security source, who also declined to be named. The group has acquired a range of bombs, mortars, rockets, anti-tank and anti-aircraft missiles, Hamas officials have said.

The expanding capabilities have produced increasingly lethal results over the years. Israel lost nine soldiers during its incursion in 2008. In 2014, the number jumped to 66.

Hamas, whose 1988 founding charter called for Israel's destruction, is classified a terrorist organisation by Israel, the United States, the European Union, Canada, Egypt and Japan. The group's leaders are spread across the Middle East in countries including Lebanon and Qatar, but its power base remains Gaza.