Gaza has come under renewed bombardment from Israel, as agreement on a ceasefire to end a week of violence remains elusive.
A number of explosions overnight left parts of the territory without power.
Earlier, US Secretary of State Hillary Clinton arrived in Israel to support peace moves being brokered in Cairo.
At least 20 Palestinians were reported to have been killed on Tuesday. Two Israelis - a soldier and a civilian - were killed in rocket strikes.
Egyptian and Palestinian officials had said on Tuesday that a ceasefire would shortly be announced at talks in Cairo.
But Israeli spokesman Mark Regev told the BBC a deal was not yet done.
"I have no doubt that Hamas would be more than happy to have a temporary respite - a time out... so they could rest and re-arm and we would have missiles on Israel next week or next month. We are not interested in that," he said.
A senior Hamas official, Izzat Risheq, said a deal might not be reached until later on Wednesday.
In the early hours of Wednesday, the BBC's Lyse Doucet in Gaza tweeted "very loud explosions - electricity out".
The BBC's Rushdi Abualouf, also in Gaza, said eight missiles had destroyed the main complex of a Hamas-run government department in the centre of Gaza City and the shattered building was on fire. He said a large part of Gaza had been blacked out.
International journalists covering the conflict also reported a loud explosion that blew in windows at the Deira Hotel.
A few hours earlier, French news agency AFP said a building in Gaza City housing its office was hit by an Israeli air strike. No staff members were injured.
The Israeli military later tweeted it had "surgically targeted a Hamas intelligence operations centre" on the seventh floor of a media building.
On Tuesday it had asked residents in villages around Gaza City to move to central areas for their own safety.
Call for de-escalation
Mrs Clinton, speaking alongside Israeli Prime Minister Benjamin Netanyahu, said she would work with Israel and Egypt on brokering a truce in Gaza "in the days ahead".
She added that it was "essential to de-escalate the situation" in Gaza and welcomed Egyptian mediation efforts.
Mr Netanyahu said Israel would welcome a diplomatic solution but warned that he was ready to take "whatever action" was necessary.
After her late-night talks with Mr Netanyahu, Mrs Clinton is due to travel to Ramallah in the West Bank and Cairo on Wednesday.
The Israeli army said about 150 rockets had been fired from Gaza towards Israel on Tuesday, and many had been intercepted by Israel's Iron Dome missile defence system.
It also said one of its soldiers had been killed in the Eshkol area by a rocket - the first military fatality in the recent upsurge.
An Israeli civilian working for the defence ministry was killed by another strike in the Negev desert.
Among those killed in Gaza on Tuesday were two journalists from a Hamas-linked TV channel.
Separately, Hamas fighters summarily executed six people on Tuesday afternoon, accusing them of being Israeli informers.
One eyewitness told AFP news agency: "Gunmen in a minibus pulled up in the neighbourhood, pushed six men out and shot them without leaving the vehicle."
TV images showed one of the victims being dragged through the streets by a motorcycle with armed men riding alongside.
In another development, Turkish Prime Minister Recep Tayyip Erdogan accused Israel of "ethnic cleansing" in Gaza.
Addressing MPs of his governing AK Party on Tuesday, he said Israel's bombardment could not be considered self-defence.
Ties between Israel and Turkey have been strained since Israeli commandos boarded a Gaza-bound aid ship in 2010, sparking clashes that left nine Turkish activists dead.
Israel launched its offensive with the killing of a Hamas military leader last Wednesday. Israel says that assassination, and its subsequent offensive, aims to end rocket fire from Gaza.
Gaza's health ministry says at least 130 Palestinians, many of them civilians, have died since last Wednesday. Three Israeli civilians died last Thursday in the Israeli town of Kiryat Malachi.
The contents of the ceasefire plan brokered by Egypt are not known, but both Israel and Hamas have presented conditions.
Israel's demands include no hostile fire of any kind from Gaza and international efforts to prevent Hamas from re-arming, while Hamas is demanding an end to the blockade on Gaza and targeted killings by Israel.
Israeli troops are massed along the border, raising fears of a ground offensive similar to that of 2008-09.
UN Secretary General Ban Ki-moon met Arab League Chief Nabil al-Arabi in Cairo on Tuesday, before heading for Israel.
During a joint news conference with Mr Netanyahu, Mr Ban condemned Palestinian rocket attacks but urged Israel to show "maximum restraint".
"Further escalation benefits no-one," he said.