David Cameron has urged Israel to "do everything possible" to end the crisis in Gaza as its military continued to trade missile strikes with Hamas amid fears of a new ground war.
In a phone call with Israeli counterpart Benjamin Netanyahu last night, he expressed sympathy for the continued "unacceptable" rocket attacks by militants which have reached Jerusalem for the first time.
A Downing Street spokeswoman said: "The Prime Minister expressed his sympathy for the unacceptable rocket attacks that Israel continued to suffer.
"He also expressed concern over the risk of the conflict escalating further and the danger of further civilian casualties on both sides.
"The Prime Minister said that the UK was putting pressure on both sides to de-escalate and urged Prime Minister Netanyahu to do everything possible to bring the conflict to an end."
Mr Cameron's call came amid intensified international diplomatic efforts to prevent the violence escalating into a full-scale conflict.
Israel launched around 300 airstrikes yesterday on targets in the Gaza strip, including the Hamas prime minister's HQ, a police compound and a building used by Hamas for media broadcasts.
Tens of thousands of reserves have been mobilised while large numbers of troops and tanks have been stationed along the border as it considers whether to launch a ground offensive into Gaza.
Israel said it still wants to hit hundreds of targets.
Palestinian militants meanwhile raised the stakes by aiming some of the latest salvo of 100 rockets at the cultural and commercial centre of Tel Aviv as well as the Israeli capital Jerusalem.
The Israeli military said the onslaught raised the total number to roughly 500 since this week's fighting began.
Health officials said 48 Palestinians have been killed and more than 400 civilians wounded since Wednesday. Three Israeli civilians were killed earlier in the week and more than 50 have been wounded.
Egypt stepped up efforts to broker a ceasefire - holding talks in Cairo with Hamas and key allies Qatar and Turkey while the Arab League also convened an emergency meeting.
Labour urged United Nations Secretary General Ban Ki-moon to step in and lead a "full-scale diplomatic initiative" in the region within the coming days.
Shadow foreign secretary Douglas Alexander said: "We urge the UN Secretary General to visit the region this week to begin talks with all parties, and with partners in the region.
"The only hope for peace and security for the citizens of the region will be through re-starting the stalled negotiations towards agreeing a two state solution."
Children's charity Unicef appealed for the "utmost restraint" from both sides to prevent youngsters being killed and injured.
Six Palestinian children aged between 10 months and 15 years have been reported killed and 60 injured in airstrikes on Gaza, it said, with another fatally wounded by a rocket that fell short.
And Israeli schools within a 25-mile radius were closed because of the "indiscriminate" rocket attacks.
"Both rocket attacks and airstrikes are putting children and their families at risk, leaving them exposed to physical harm and mental distress," it said.
"Unicef calls on all parties to do everything to exercise the utmost restraint and to protect the rights and well-being of all children."
Meanwhile protesters gathered near the Israeli embassy in London yesterday condemned the British Government's stance on the conflict.
Speakers took to a podium to condemn the Government after Foreign Secretary William Hague said the Hamas regime in Gaza bore "principal responsibility" for the escalation of violence.