Britain called for an "immediate halt to all violence" in Gaza as Israel's devastating airstrikes continued.
As fears of a ground offensive grew, Foreign Secretary David Miliband said an "urgent ceasefire" was needed to stop "massive loss of life" in the territory.
He insisted that Tel Aviv must abide by its "humanitarian obligations", and Prime Minister Gordon Brown shared his "grave concern" over the situation.
At least 290 people have now been killed since F-16 bombers started carrying out strikes on the highly populated Gaza Strip on Saturday, according to Palestinian emergency services. Hundreds more have been wounded.
Israel said key security sites operated by Hamas militants were being targeted in a bid to stop rockets attacks - which have escalated sharply since a six-month ceasefire agreement expired last week. However, many civilians and children have been caught up in the carnage.
Mr Miliband said: "The Prime Minister and I are following developments in Gaza with grave concern. The rise in rocket attacks on Israel since December 19, and yesterday's (Saturday) massive loss of life, make this a dangerous moment which should be of concern to the whole of the international community.
"The UK supports an urgent ceasefire and immediate halt to all violence."
After four hours of emergency talks, the UN Security Council upped the pressure on Tel Aviv on Sunday morning by delivering a plea for the bloodshed to stop.
A statement made no direct reference to actions by either side, but said: "The members of the Security Council expressed serious concern at the escalation of the situation in Gaza and called for an immediate halt to all violence. The members called on the parties to stop immediately all military activities."
However US ambassador Zalmay Khalilzad stressed that Hamas held the key to restoring calm. "We believe the way forward from here is for rocket attacks against Israel to stop, for all violence to end," he said.
