Kamal Sweleim’s family has owned a farm in this northern part of Gaza for six decades. For most of that time, it was a mix of citrus orchards and plump cows, and the family made a handsome living selling its products to Israel, Jordan and the West Bank.

But 10 years ago, when the second Palestinian uprising broke out, spreading violence in Israeli streets, Israeli tanks started repeatedly tearing through the family’s fields, chasing militants. Last year, during the Israeli war in Gaza, the Sweleims were ordered to move out, and their trees and wells were bulldozed.

A once prosperous clan with good ties to Israel, they now rent a tiny house, living off cousins and international welfare. “Don’t remind me of what we used to have,” Mr. Sweleim said as he stood near his desolate fields surrounded by destroyed houses. “My father would never believe where we have ended up.”

A United Nations report issued on Thursday says the Sweleims are part of the 12 percent of the population of Gaza — 178,000 people out of 1.5 million — who have lost livelihoods or have otherwise been severely affected by Israeli security policies along the border, both land and sea, in recent years. These include the establishment of “no-go zones” and frequent incursions.

The report estimates that the restricted land comprises 17 percent of Gaza’s total land mass and 35 percent of its agricultural land. Israel also restricts Gazan fishing to three nautical miles offshore. Catches are greatly reduced, leading some fishermen to take a long, risky sail into Egyptian waters to buy fish from Egyptian fishermen and return home to sell it.

The study, issued by the Office for the Coordination of Humanitarian Affairs in the occupied Palestinian territory, says that anti-Israeli militants operate from the border areas in question, planting explosive devices, firing at Israeli military vehicles and shooting rockets and mortar rounds at civilians.

But it argues that Israel has an obligation under international law to protect civilians and civilian structures. It also notes that Israel has never clearly told those living in the area where they may and may not live and operate.

“The Israeli military has consistently failed to provide the affected population with accurate information about the main parameters of the access regime being enforced, particularly in the farming areas, and to a lesser degree in the restricted fishing areas,” it said.

It added that Israel “has failed to physically demarcate the restricted areas in any meaningful way, even though it carries out land incursions into the restricted areas three to four times every week and naval forces continuously patrol the coast.”

Last year, the Israeli Air Force dropped leaflets telling Gazans they could not come within about 990 feet of the border. But, the report says, the restricted area is about 3,300 to 4,950 feet.

Lt. Col. Avital Leibovich, a spokeswoman for the Israeli military, said that Hamas, the Islamist group that has ruled in Gaza for the past few years, knowingly endangers the civilians living near the border by sending militants there.

“What is foremost in our minds is protection of our civilians who live within range of the border,” she said by telephone when asked about the new study. “If your choice is to operate terror, you have to bear the consequences.”

Colonel Leibovich said there had been an increase in militant activity from inside the border over the past 18 months, with eight attacks since January.

“This is also why it seems like we are changing, because the activity has been stepped up from the other side and we need to deal with it,” she said.

The United Nations says its report is based on more than 100 interviews and focus group discussions carried out in March and April and complemented with quantitative data. It estimates that damage done by Israel to border farms and property over the past five years amounts to about $308 million. Fruit trees, greenhouses, sheep and chicken farms, and water wells account for most of this.

The loss to fishermen over the same period was put at $26.5 million.

For advocates of coexistence, a particularly sad aspect of the findings is that many of those most affected by Israeli security practices, like the Sweleim family, once had the closest relationships with Israel, selling their goods there and often working there. They were fairly well off and many were politically moderate. Today, the group is among the most isolated and depressed, with relatives and friends reluctant to visit for fear of attack.