Hamas says the wife and child of its military commander, Mohammed Deif, have been killed in an Israeli air strike on the Gaza Strip.
At least 19 Palestinians have died since hostilities resumed on Tuesday, with both sides blaming each other for the collapse of the Cairo peace talks.
The Israeli military said it had carried out 92 air strikes in response to 137 rockets fired at its territory.
Six weeks of fierce fighting have left at least 2,103 people dead.
Egypt has expressed "profound regret" at the end of the 10-day period of calm and said it will continue trying to secure a lasting truce.
It is believed the air strike on a house in Gaza City late on Tuesday that killed Mohammed Deif's wife and their young son was intended to kill the militant himself, reports the BBC's Kevin Connolly in Jerusalem.
A Hamas official told the AFP news agency that Mr Deif was "still alive and leading the military operation" against Israel.
The commander of Hamas' armed wing, the Izz al-Din al-Qassam Brigades, has survived a number of previous Israeli assassination attempts believed to have left him with severe disabilities.
Israeli Interior Minister Gideon Saar said the attack was justified because Mr Deif was "personally responsible" for dozens of deaths.
Yaakov Perry, Israel's science minister and former security service chief, said he was "convinced that if there was intelligence that [he] was not inside the home, then we would not have bombed it".
Rescue workers later pulled out of the remains of the house the bodies of three members of the family that lived there, medics said.
Kevin Connolly, BBC News, Jerusalem
No-one is rushing to make an official declaration that the Cairo peace talks are dead but it is hard to see what could now bring them back to life.
Hamas blames Israel for the end of the ceasefire just as Israel blames Hamas - the one point of agreement between them appears to be that the time for talking has gone at least for now.
Any diplomatic failure in the Middle East can be explained by the formula that the maximum one side was prepared to offer was less than the minimum that the other side was prepared to accept. Never has it been truer than it is now.
Both sides went in to this round of negotiations with what diplomats would call maximalist positions - Hamas demanding an opening of Gaza's borders, Israel aspiring to the demilitarisation of Gaza.
But neither side can afford to allow any concessions which might be portrayed as a victory for the other. In neither society would public opinion stand for it.
That is a recipe for stalemate - and at times of stalemate both sides are inclined to turn back to their weapons of war.
There were further Israeli air strikes on Gaza on Wednesday which, according to local medics, killed a heavily pregnant woman and several children.
Our correspondent says the apparent attempt to kill Mohammed Deif may explain the intensity of the rocket fire that came after the collapse of the Cairo peace talks.
Air-raid sirens sounded in many towns and cities in southern and central Israel, including Tel Aviv and Jerusalem, as 50 rockets were launched on Tuesday and 30 on Wednesday.
The Israeli government accused Hamas of breaking the ceasefire by launching a salvo of rockets about eight hours before it was to have expired, and told its delegation in Cairo to return home shortly afterwards.
Palestinian negotiators blamed Israel for the failure of the indirect talks.
"Israel thwarted the contacts that could have brought peace," said Azzam al-Ahmed, a senior member of the Fatah movement of Palestinian President Mahmoud Abbas.
However, Israeli government spokesman Mark Regev rejected the accusation, saying rockets from Gaza were "a clear violation of the ceasefire" and "destroyed the premise upon which the talks were based."
Israel has been seeking guarantees that Hamas and other factions in Gaza would be disarmed, while the Palestinians were demanding an end to the Israeli and Egyptian blockades of Gaza, and the establishment of a seaport and airport.
UN Secretary General Ban Ki-moon said he was "gravely disappointed by the return to hostilities".
Meanwhile, human rights groups have called on Israel to allow them into Gaza so they can investigate allegations of violations of international humanitarian law by both sides.
In a joint statement, Amnesty International and Human Rights Watch accused Israel of "playing bureaucratic games" with them by continually denying the groups' requests to cross the border.
Israel launched Operation Protective Edge on 8 July with the aim of ending Hamas rocket fire. It also sought to destroy tunnels dug under the frontier with Israel used by militants to launch attack.
Since then, at least 2,036 people, most of them civilians, have been killed in Gaza, according to the Palestinian health ministry. The Israeli authorities say 64 Israeli soldiers have been killed, along with two Israeli civilians and a Thai national.
Hamas military commander Mohammed Deif
- Leader of Izz al-Din Qassam Brigades since 2002
- Top of Israel's most wanted list for years
- Born in Khan Younis in early 1960s
- Spent time in Israeli jail in 1989
- Mentored by Hamas bomb maker Yehya Ayyash in 1990s
- Blamed for dozens of deaths in spate of suicide bombings from 1996
- Seen as mastermind of strategy to build tunnel network under Gaza
- Lost an eye and limbs in earlier assassination attempts