Children in Gaza risk regular gunfire from Israeli troops while looking for construction material in destroyed buildings close to the blockaded enclave's border with Israel, Save the Children said on Tuesday.

Citing a report by a UNICEF-led group on children caught up in conflict, it said 26 children were shot by Israeli soldiers near the border last year, including 16 outside the buffer zone which extends 300 metres (yards) from the border security wall and fence.

The children's charity also said in September two 16-year-old boys and their 91-year-old grandfather were killed by an Israeli tank shell some 700m from the border.

Israel says Palestinian militants in the past have been detected trying to plant explosives in the buffer zone, in the hope of harming military patrols.

Every day, thousands of Gazans, mainly young men and boys, venture close to the buffer zone with their donkey carts, hoping to earn some money from the wreckage of factories and houses flattened by Israel in its conflict with Islamist Hamas militants who seized control of the Gaza Strip in 2007.

"Because of the blockade's devastating economic impact, children are being forced to work and scavenge near the fence. Even those who are not in the so-called "buffer zone" unilaterally imposed by Israel are being targeted by Israeli soldiers," Chris Gunness, spokesman for the U.N. agency in charge of Palestinian refugees, said in a statement.

Israeli officials were not immediately available for comment.

Two years after Israel's military assault in Gaza to force Hamas to stop firing rockets into its towns, aid workers say construction material remains in high demand because of Israel's restrictions on imports of steel and cement into the territory.

Israel fears Hamas will use such materials to make weapons but the lack of building materials has driven up prices and means thousands of homes damaged during the Israeli offensive and other incursions have not been repaired.

"Until now we can see with our own eyes people living in tents two years after the offensive in Gaza, schools that have not been rehabilitated and also children scavenging near the border to offer their families some income to feed themselves," Save the Children spokesman Osama Damo said by telephone from Gaza.

Despite the risk to their lives, many children continue to collect scrap because they believe they have no alternative, seeing rampant unemployment all around them, he added.

Save the Children also said it had reports that children were dropping out of school to take up whatever work they could find.