He said that an air strike on a house in the southern Gaza town of Deir-el-Balah killed a woman of 26 and another 23-year-old pregnant woman, as Israel pressed on with its 18-day campaign to stamp out Gaza rocket fire.
The baby was saved, he said.
Two other people wounded earlier in shelling of the southern city of Khan Yunis, died of their injuries, he said, bringing the total number of Gazans killed in the Israeli campaign to 832. A total of 36 people have been killed on the Israeli side.
Nearly 100 Palestinians were killed on Thursday, according to Qudra, one of the bloodiest days of the conflict.
Projectiles fired into Israel have killed three civilians -- two Israelis and a Thai migrant worker -- and fighting in and around Gaza has killed 32 Israeli soldiers.
WATCH: UN school in Gaza hit by Israeli strike
It is the bloodiest conflict in the besieged Palestinian territory since a 2009 military operation there.
NGOs put the Palestinian civilian toll at around 80% of total casualties, including a large number of women and children.
"We are gravely concerned by the ongoing heavy level of civilian causalities," British foreign secretary Philip Hammond said at a press conference in Jerusalem with Israeli Prime Minister Benjamin Netanyahu, at which he also defended Israel's right to self-defence in the face of a conflict triggered by Hamas rocket fire.
Fifteen people were killed Thursday when Israeli fire hit a UN shelter in Gaza.
Emergency services spokesperson Ashraf al-Qudra said at least 15 people had been killed and 200 wounded by Israeli shelling of a school run by the UN agency for Palestinian refugees (UNRWA) in the northern town of Beit Hanun, where hundreds of civilians had sought refuge from the violence.
He gave no immediate details of those killed, but an AFP correspondent reported that a mother and her one-year-old infant were among the dead brought into a nearby mortuary.
Earlier Thursday, US airlines lifted a flight ban to Israel, with other international airlines expected to follow suit.
The US national aviation agency had imposed the restriction on Tuesday after a rocket hit a house very close to the runways, in a move mirrored by Europe.
It was renewed late on Wednesday, prompting Hamas, the de facto power in Gaza, to hail the suspension of Tel Aviv flights as a "great victory for the resistance."
US secretary of state John Kerry Thursday pressed efforts to end bloodshed in Gaza, reaching out to allies of Hamas as the Islamist movement's war with Israel raged into a 17th day.
Kerry -- who is in Egypt, which has drafted a truce proposal for the Israel-Hamas conflict -- spoke by phone with the foreign ministers of Qatar and Turkey, a US official said.
Hamas chief Khaled Meshaal is based in Qatar, while Turkey's Islamist-oriented Prime Minister Recep Tayyip Erdogan has harshly criticised Israel's assault on Hamas-ruled Gaza as well as Egypt's role in trying to clinch a ceasefire.
A US official said Kerry was hoping Qatar and Turkey would use their influence to encourage Hamas to accept a ceasefire plan, which the Islamist group has so far rejected.
Meshaal vowed late on Wednesday there would be no end to the fighting without the lifting of Israel's eight-year blockade on Gaza.
Khuzaa under fire
There was no let-up to the violence in Gaza, however, with most of Thursday's 51 victims killed in and around Khuzaa, a flashpoint area east of Khan Yunis which has been the site of intensive fighting since Tuesday.
Gaza's health ministry issued a call for international protection for civilians in the area, saying anyone leaving home was being targeted by Israeli fire.
On Wednesday, the Red Cross and Palestinian ambulances managed to evacuate 150 people from the area following negotiations with both sides, and another convoy of 10 ambulances entered the area early on Thursday, an ICRC spokesperson told AFP.
In Jabaliya refugee camp, residents gathered at first light to examine the damage after an air strike destroyed houses and a mosque.
Netanyahu said Israel was doing everything it could to minimise casualties, pinning the blame on Hamas for its "grotesque (and) inhuman" use of civilians as human shields.
Hammond then flew to Cairo where he was due to hold talks with Egyptian President Abdel Fattah al-Sisi.
UN chief Ban Ki-moon was also in Egypt and Israel earlier this week, in the hope of hammering out a regional truce deal, with Kerry acknowledging there had been "some progress in moving toward that goal."
On a whirlwind visit, Kerry on Wednesday met Palestinian president Mahmud Abbas in the West Bank city of Ramallah then held a two-hour meeting in Tel Aviv with Netanyahu before flying back to the Egyptian capital.
So far, 32 Israeli soldiers and three civilians have died in the fighting, one of whom was a Thai farm labourer who was killed when a rocket struck the greenhouse where he was working in southern Israel.
Following his death, Bangkok demanded Israel "immediately" relocate 4,000 Thai nationals working near the Gaza Strip to areas safe from the fighting, a foreign ministry spokesperson said.
(With inputs from Reuters, AFP and AP)