David Cameron and Barack Obama have expressed "serious concern" about renewed hostilities in Gaza and pleaded for another ceasefire.
The Prime Minister and US President condemned Hamas for launching rockets from civilian areas, and stopped short of directly criticising Israel's response.
But, in a telephone call, the leaders did agree that Tel Aviv needed to show "restraint" and take "utmost care" to avoid innocent casualties.
The discussions came as violence intensified after the three-day ceasefire came to an end. At least five people were reportedly killed in air strikes on Gaza, while Palestinian militants have resumed their rocket fire.
Tensions have also been growing in the West Bank, where protesters have clashed with Israeli troops.
The UK, France and German foreign ministers also joined together to release a statement saying they were "very concerned" over the fresh fighting and calling for an immediate return to a ceasefire.
The statement, on behalf of Philip Hammond, Frank-Walter Steinmeier of Germany and French Foreign Minister Laurent Fabius, said: " All parties should take immediate practical steps to address the urgent humanitarian needs including to facilitate full humanitarian access.
"We are ready to support the establishment of a sustainable ceasefire and have already made proposals to this end.
"We will work further with all our partners on this issue. To be sustainable, a ceasefire must envisage steps to address both Israeli security concerns and Palestinian requirements regarding the lifting of restrictions on Gaza.
"The final objective must be to see a return to talks on a negotiated two-state solution, which remains the only way to resolve the conflict and end the human suffering it causes once and for all."
Earlier, former Foreign Office minister Baroness Warsi spelt out her long-standing frustration with Government policy towards Israel.
The Tory peer, who dramatically quit this week in protest, indicated that her objections went beyond the Prime Minister's refusal to condemn Tel Aviv over the civilian death toll.
Repeating her demand - backed by the Liberal Democrats and Labour - for an immediate suspension of arms exports to Israel, Lady Warsi said: " Our language was not there. It was lagging behind. I don't think it was just words that would have stopped me from doing what I did.
"I think it was a combination of issues. It was the language that we were using, I think it was our lack of support for international justice and accountability for the crimes that had been committed."
She said the Government needed to "move towards a Middle East policy that is in the long term sustainable" and she cited the UK's decision to abstain when the UN General Assembly voted in November 2012 to recognise a Palestinian state as symptomatic of the problems.
"There is no point in us talking about a two-state solution if we don't do the simple things like recognising Palestine in the way that the majority of the world has at the UN," she added.
Lady Warsi said her resignation meant she could "live with myself", and she hoped it would bring together a "broad coalition" to address the issues behind the conflict.
Giving details of the conversation between Mr Cameron and Mr Obama, a Downing Street spokesman said: "They noted that Hamas had started firing rockets into Israel once again and condemned that they are launching these from deep within civilian neighbourhoods, putting innocent people at risk.
"They agreed that Israel has a right to defend itself but it should do so in a way that exercises restraint and Israeli forces must take utmost care to avoid civilian casualties.
"Both agreed that the priority must be to re-establish a ceasefire that paves the way for negotiations on a more lasting peace that allows both Israelis and Palestinians to live in safety alongside one another."
A joint statement released by Britain, France and Germany said: "We call upon all parties immediately to return to a ceasefire. We fully support the ongoing efforts by Egypt to this end.
"All parties should take immediate practical steps to address the urgent humanitarian needs including to facilitate full humanitarian access.
"We are ready to support the establishment of a sustainable ceasefire and have already made proposals to this end. We will work further with all our partners on this issue.
"To be sustainable, a ceasefire must envisage steps to address both Israeli security concerns and Palestinian requirements regarding the lifting of restrictions on Gaza."
Meanwhile, the Disasters Emergency Committee (DEC) said its appeal for Gaza had raised over £4.5m in less than 24 hours.
DEC chief executive Saleh Saeed said: " The funds are desperately needed, with on-going fighting in Gaza creating an unbearable situation for families and children.
"Hundreds of thousands of people have been forced to leave their homes, the healthcare system is on the verge of collapse and many people have little or no clean water.
"Despite the end of the ceasefire, aid is getting through and many of our member agencies are still working on the ground.
"But with the scale of the humanitarian crisis in Gaza, they urgently need more money to scale up their life-saving work."