Israeli soldiers are routinely shooting at Gazans well beyond the unmarked boundary of the official 300 metre-wide no-go area, rights groups say.

According to Bassam Masri, head of orthopaedics at the Kamal Odwan hospital in Beit Lahiya in the north of Gaza, about 50 people have been treated for gunshot wounds suffered in or near the buffer zone while collecting rubble in the past three months; about five have been killed.

He estimates that 30% of the injured are boys under 18.

Defence for Children International (DCI) has documented 10 cases of children aged 13 to 17 being shot in a three-month period between 50 and 800 metres from the border. Nine were shot in a leg or arm; one was shot in the stomach.

The creation of the no-go area has forced farmers to abandon land and residents to leave homes for fear of coming under fire. Last month a 91-year-old man and two teenage boys were killed while harvesting olives outside the official zone when Israeli troops fired shells. Forty-three goats also died in the attack.

In another case a mother of five was killed by a shell outside her home near the zone in July.

Israel declared the buffer zone inside Gaza after the three-week war in 2008-9, saying it was intended to prevent militants firing rockets. It has dropped leaflets from planes several times warning local people not to venture within 300 metres of the fence that marks the border or risk being shot.

However, the UN, aid agencies and rights groups say that Israel has unofficially and without warning extended the zone to up to 1km from the fence, leaving residents and farmers uncertain whether it is safe to access their land or property.

"The army knows the kids are there to collect. They watch them every day and they know they have no weapons," said Mohammed Abu Rukbi, a fieldworker with DCI. "They usually fire warning shots but the kids don't take much notice."

Mohammed Sobboh, 17, was shot just above the knee on August 25 when he was 800 metres from the border, he said. The 12 people in his family have no other income and are not entitled to aid from the UN as they are not refugees.

Israeli soldiers shot dead a horse and a donkey used by Mohammed and his brothers to carry the rubble, he said.

His brother, Adham, 22, said children as young as eight collect debris from former settlements and demolished buildings for 30-40 shekels (£5.20-£7) a day. "The price has gone down because a lot of people are collecting," said Adham.

According to Dr Masri, the number of shootings has increased as more impoverished Gazans turn to collecting rubble to sell as construction material, which is still under Israeli embargo. "Every day we have one or two cases. Some kids are facing permanent disability. Most of the injuries are to the legs and feet, suggesting the soldiers did not aim to kill. That means they know that the people aren't militants."

Ziad Tamboura, 27, lying in a hospital bed with a heavily bandaged foot, was shot last week while collecting 500 metres from the border. X-rays showed the bones in the foot to be smashed by the bullet. He collected rubble in order to feed his wife and child. "If I am able to walk again, I will go back. There is no other work."

The Gaza City-based Al-Mezan Centre for Human Rights is to mount a legal challenge jointly with the Israeli groups Adalah and Physicians for Human Rights to breaches of the official buffer zone. "The area [the Israelis] announced is not the same as what exists on the ground," said the centre's Samir Zaqout.

He criticised the Israelis for shooting and shelling unarmed civilians. "They know everything. They have the technological capacity to monitor the area. They have drones in the sky all the time. They are observing and screening everything."

According to the UN, about 30% of Gaza's arable land is contained within 300 metres of the 50km border. The difficulty farmers face in reaching their land had had an impact on the availability of crops in Gaza, Zaqout said. "Tomatoes are now 10 shekels a kilo, whereas the price used to be one or two shekels."

The Abu Said family, whose land lies outside the buffer zone, felt confident that their faces were well known to Israeli troops monitoring the area. "Every day six or seven members of my family are there [on the land]," said Mohammed Abu Said.

But on 12 September, 91-year-old Ibrahim Abu Said, his 17-year-old grandson, Hussam, and a family friend, Ismail Abu Owda, 16, were killed by a shell fired from a tank on the Israeli side of the border. "This was a very old man taking care of his goats," said Mohammed, Ibrahim's son. "Our land used to be like a heaven. Now it's like a desert."

He blamed Palestinian militants for firing rockets as well as the Israeli military.

In a statement, the Israeli military said the 300-metre buffer zone was created in response to "many incidents of hostile terrorist activity" close to the security fence, often made "under a civilian disguise".

It added: "The IDF acts in order to prevent harm to civilian populations in its operations and any complaint expressed regarding its soldiers' conduct will be … examined according to the existing policy."

In the firing line

Children shot in "buffer zone" while collecting rubble

Mohammad, 17, shot in left leg, 800m from border, 25 August

Khaled, 16, left thigh, 600m from border, 31 July

Hameed, 13, left arm, 50m from border, 14 July

Nu'man, 14, right leg, 300m from border, 10 July

Arafat, 16, left ankle, 50m from border, 10 July

Mohammad, 16, stomach, 500m from border, 23 June

Abdullah, 16, just above right ankle, 60m from border, 22 June

Ibrahim, 16, right leg, 400m from border, 16 June

Awad, 17, just above his right knee, 350m from border, 7 June

Hasan, 17, just below right knee, 300m from border, 22 May

Source: Defence for Children International