A mother of five was killed by Israeli artillery fire when she went to fetch her two-year-old son from outside her village home close to the "buffer zone" created by Israel along its border with Gaza.

Three of her relatives were wounded in the shelling earlier this week, but Red Crescent ambulances were not permitted to reach the family for several hours.

According to the woman's husband, Nasser Abu Said, 37, the attack began without warning at about 8.30pm on Tuesday with two shells being fired as the family of 17 sat outside their house in the village of Johar a-Deek. Apart from Nasser and his 65-year-old father, the entire group was women and children.

"It was completely quiet, there were no rockets being fired or we wouldn't have been sitting outside," he said, referring to Qassam missiles launched by militants into Israel.

His sister and his brother's wife were injured by shrapnel. The family moved indoors and called an ambulance. "About 10 minutes later the ambulance called back to say the Israelis had refused them permission to come to the house," said Nasser.

His wife Ne'ema, 33, soon realised their youngest son, Jaber, was not among the children she was attempting to calm down, and was probably asleep on a mattress outside that he often shared with his grandfather.

As she went to fetch the toddler, another shell landed. "I called to my wife three times," said Nasser, who realised his father had also been badly injured in his leg and stomach. "I could hear small noises coming from her. I knew she was dying."

Via Palestinian co-ordinators, the IDF told the family that anyone going outside the house would be shot dead. Nasser began to tend to his injured father, knowing he could not reach his dying wife.

"I was holding myself in, especially in front of the children," he said. The children were crying hysterically and some had wet themselves, he added.

After two hours, an ambulance was allowed to reach the family. The Palestinian Centre for Human Rights (PCHR), which investigated the incident, said Ne'ema and her wounded relatives were taken to al-Aqsa Martyrs hospital in Deir al-Balah, where it was confirmed she had died from shrapnel wounds.

The Israeli Defence Force (IDF) said it had identified a number of suspects close to the border. "An IDF force fired at the suspects and identified hitting them," it said. The incident was being investigated, it added, but declined to say why ambulances had not been allowed to reach the family.

Since the three-week war in Gaza that began in December 2008, the IDF has continued to fire on Palestinians it suspects of launching rockets at Israeli civilians or attempting to attack Israeli forces. It created a 300m-wide buffer zone on Palestinian farmland adjacent to the border with Israel and warned it would shoot anyone seen within the forbidden area.

The Abu Saids say their land is not used by militants to fire rockets as it is open ground in full view of an Israeli watchtower at the border 400m away.

In the first five months of this year, 22 Palestinians were killed by Israeli forces in the buffer zone, according to the PCHR. The IDF says one soldier and a Thai farmworker were killed and two soldiers lightly wounded in militant attacks in the first half of this year.

Palestinians have been unable to harvest their crops in the zone, which has swallowed about 30% of Gaza's arable farmland. The Abu Said family have lived in the area for 40 years, but have had to abandon the part of their land inside the zone. "Everyone is afraid to come to this house," said Nasser.

The house, isolated down a rutted track, was riddled with shrapnel damage from Tuesday's shelling, and dried blood still lay in the sand where Ne'ema had been killed.

The PCHR condemned the shelling which, it said, "constitutes the highest degree of disregard for Palestinian civilians' lives". This was not an isolated incident but "part of a series of continuous crimes committed by the [Israeli military]".