Israel has been shelling Gaza for a fifth day, killing six Palestinians, as Prime Minister Benjamin Netanyahu said Israel was ready to "significantly expand" its operation in Gaza.
Rockets from Gaza have again landed in Israel. One rocket caused damage and injuries in the city of Ashkelon.
In Gaza, two media buildings were hit and eight Palestinian journalists hurt.
Sources on both sides have confirmed to the BBC that attempts to reach a ceasefire are continuing.
At least six people were killed in Gaza on Sunday, doctors said, including two children from the same family.
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 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 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.People around Tel Aviv heard the air raid sirens sound again on Sunday but no rockets from Gaza landed there
Two rockets fired from Gaza were intercepted and destroyed by the Iron Dome missile shield, police say.
However, a rocket from Gaza made a direct hit on a residential building in Ashkelon, causing injuries and damage.
Health officials in Gaza say at least 52 Palestinians - including 11 children - have been killed since Israel launched Operation Pillar of Defence on Wednesday with an air strike that killed Hamas military chief Ahmed Jabari. Three Israelis were killed on Thursday.
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.
Fresh air raids were also heard across Gaza City throughout the night and into Sunday.
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.
Egyptian President Mohammed Mursi said an Israeli ground invasion would have "serious repercussions", saying Egypt would never accept it "and neither will the free world". The BBC's Wyre Davies in Gaza: "The longer the attacks go on, the fewer supplies hospitals have"
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.
'Back to the Middle Ages'
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.