Israel has launched more deadly strikes on Gaza, with PM Benjamin Netanyahu saying Israel was ready to "significantly expand" its operation.
At least nine people were killed in one strike on the house of a Hamas official in Gaza City, BBC correspondents say.
Gaza militants continue to fire rockets at Israel, with injuries reported in towns including Ashkelon and Ofakim.
Sources on both sides say attempts to reach a ceasefire after five days of violence are continuing.
US President Barack Obama, speaking on Sunday, said Washington was "fully supportive of Israel's right to defend itself".
However, UK Foreign Secretary William Hague warned that a ground invasion would "lose Israel a lot of the international support and sympathy they have in this situation".
Speaking to Sky News, Mr Hague stressed that the Hamas movement, which governs Gaza, bore "principal responsibility" for the current conflict.
'Prepared for expansion'
Israeli soldiers were ready "for any activity that could take place", Mr Netanyahu said at a cabinet meeting on Sunday.
"We are exacting a heavy price from Hamas and the terrorist organisations and the Israel Defense Forces (IDF) are prepared for a significant expansion of the operation," he said.
The Israeli military confirmed to the BBC that it knew foreign journalists were in at least one of the two media buildings it struck in Gaza, but said they were not the target.
All the sites hit overnight were "positively identified by precise intelligence over the course of months", it said on Twitter.
One of the eight injured Palestinian journalists had to have a leg amputated.
The Middle East Foreign Press Association has issued a statement expressing concern at the air strikes on the media buildings.
Among those using the buildings were a Hamas television channel, al-Quds TV, as well as Sky News and ITN. The BBC had its offices in one of the buildings until last year.
The Israeli military said it hit specific equipment - including an antenna - in the buildings, leaving the rest untouched.
But the Russian TV network Russia Today said its office in one of the buildings was damaged.
Sirens sounded in Tel Aviv, Israel's commercial capital, on Sunday for the fourth day running.
Two rockets fired from Gaza were intercepted and destroyed by the Iron Dome missile shield, police say.
Israel's state radio reported that a volley of 10 rockets had been fired at Ashdod, with three falling in a residential area and seven people treated for shock.
One rocket from Gaza made a direct hit on a residential building in Ashkelon, causing injuries and damage.
Another rocket hit a car in Ofakim, causing injuries, the IDF said.
Three Israelis were killed on Thursday.
Health officials in Gaza say at least 58 Palestinians have now been killed since Israel launched Operation Pillar of Defence on Wednesday with an air strike that killed Hamas military chief Ahmed Jabari.
The officials said a number of people were still missing under rubble and the total of injured since Wednesday was now 560.
At about 02:00 (00:00 GMT) on Sunday, the BBC's Jon Donnison in Gaza City reported hearing more than a dozen shells, apparently fired from Israeli warships.
Artillery fire is generally less accurate than air strikes, he says.
The World Health Organization says hospitals in Gaza are overwhelmed with casualties and short on supplies.
Israel opened one of the main goods crossings into Gaza on Sunday to allow in several truck loads of medical supplies and other emergency equipment.
The BBC's Paul Danahar was at the Shifa hospital when injured children were brought in, one covered in blood.
He said the hospital went from organised calm to frantic chaos as doctors tried to dress wounds. One nurse broke down in a corner and colleagues tried to comfort her.
'Back to the Middle Ages'
Egyptian President Mohammed Mursi has said an Israeli ground invasion will have "serious repercussions", saying Egypt would never accept it "and neither will the free world".
Israel has put 75,000 reservists on stand-by.
The Arab League, which met in emergency session in Cairo, is sending a delegation of foreign ministers to Gaza on Tuesday.
On Saturday, Israeli Interior Minister Eli Yishai was quoted by Israel's Haaretz newspaper as saying that the goal of the operation was "to send Gaza back to the Middle Ages. Only then will Israel be calm for 40 years".
Before the recent offensive, Israel had repeatedly carried out air strikes on Gaza as Palestinian militants fired rockets across the border.
But the aerial and naval bombardment is its most intense assault on the coastal territory since Israel launched a full-scale invasion four years ago.