Prime Minister Gordon Brown has said he is "deeply concerned" after Israeli airstrikes killed at least 195 people in the Gaza Strip.
He called on Palestinian militants to halt all rocket attacks on Israel and said Israel must "do everything in its power to avoid civilian casualties".
Conservative leader David Cameron called the violence "horrific" and said both sides must show "restraint".
Aid agencies in the region have also appealed for calm.
The strikes, the most intense Israeli attacks on Gaza for decades, come days after a truce with Hamas expired.
In a statement, Israel's military said it had been targeting "Hamas terror operatives", training camps and weaponry storage warehouses.
Mr Brown said peaceful means were "the only way of reaching a lasting solution to the situation in Gaza" and called for support for the Palestinian government of Mahmoud Abbas.
"I am deeply concerned by continuing missile strikes from Gaza on Israel and by Israel's response today," he said.
"I call on Gazan militants to cease all rocket attacks on Israel immediately. These attacks are designed to cause random destruction and to undermine the prospects of peace talks led by President Abbas.
"I understand the Israeli government's sense of obligation to its population. Israel needs to meet its humanitarian obligations, act in a way to further the long-term vision of a two-state solution, and do everything in its power to avoid civilian casualties."
The call for the two countries to reach a settlement came amid signs that tit-for-tat clashes were spiralling out of control.
Mr Cameron told Sky News: "Obviously, the pictures on our television screens are pretty horrific and all civilian casualties are a matter of great regret, so I hope that both sides will show restraint."
He said "everyone understands that Israel has a right to defend herself," but insisted that violence would not lead to a resolution.
"In the end, the only progress will be political progress and a settlement between the Israelis and the Palestinians. That is what's desperately needed," he said.
The aid agencies - Oxfam, CARE International, CAFOD, Medico International and Diakonia - said the military action would only result in a "humanitarian catastrophe".
Jeremy Hobbs, director of Oxfam International, said a "genuine and lasting solution" was required and military action would only "alienate Gaza further".
"Gaza has already been paralysed by the Israeli blockade.
"A military attack on Gaza could completely destroy essential infrastructure for sewage treatment, water provision and electricity for hospitals and homes, with devastating impact on civilians.
"A military offensive will hit ordinary people, the very people we need as partners in building a better future for the region," he said.
Martha Myers, of CARE International, said: "There is no military solution to the situation and the upsurge in violence destroys hopes for peace, refuelling the cycle of violence.
"There is simply no alternative to negotiations to address the root causes of the conflict."
Describing the strikes as "disproportionate" and "unacceptable", Liberal Democrat foreign affairs spokesman Ed Davey said: "The danger is that this will spark a new spiral of violence and terror, when what is really needed is an international effort to restore the ceasefire.
"Israel knows the international community condemns the attacks by Hamas, but it should also realise from the past that this sort of mass attack will only postpone the peace Israel, and the region, needs."