Deputy Prime Minister Nick Clegg has described the shelling of a UN-run school in Gaza as evidence that the conflict between Israel and Hamas can only lead to more senseless violence.
Urging both sides to stop fighting, Mr Clegg said he was "absolutely shocked" at the killing of 15 civilians in the school, which was crowded with hundreds of Palestinians seeking refuge from Israeli shelling.
He spoke after Israel's deputy foreign minister denied knowing who was responsible for the shelling.
On a visit to a brands company in north London, Mr Clegg said: "I am absolutely shocked, as everybody is, by this terrible news, this tragic news.
"It's heartbreaking to see more innocent civilians being killed in Gaza, this time in a UN-run school.
"If the two sides of this conflict, both Hamas and Israel, need any further reminding of how tragic this conflict is, and how it will lead to nothing, it will just lead to more senseless violence, then I think we've seen that evidence overnight.
"I plead with both sides to stop, and stop now."
Fierce fighting across Gaza yesterday saw at least 119 Palestinians killed in the bloodiest day of the 17-day conflict, taking the total death toll to at least 803.
Israel has lost 32 soldiers since launching a full-scale ground war in the enclave, with two Israeli and one Thai civilian killed by rocket or mortar fire.
Mr Clegg spoke after Israeli deputy foreign minister Tzachi Hanegbi said Israel regretted all deaths in the escalating conflict, but continued to lay the blame at the door of Hamas.
Mr Hanegbi told BBC Radio 4's Today programme: "We have the knowledge about 2,000 rockets shot at us, most of them from the midst of Palestinian civilian concentrations.
"Whilst we have to take precautions to defend ourselves, sometimes there are mistakes.
"I don't know who has caused this mistake - some say it is an Israeli mistake, maybe it was a Palestinian mistake, because, you know, every six rockets Hamas is shooting at us falls in Gaza, unfortunately for them.
"I don't have any knowledge about the specific bomb that fell. We are very sorry for each and everyone's life paying the price for this tragedy."
Yesterday, Foreign Secretary Philip Hammond called for a swift ceasefire between Israel and Hamas as he joined international diplomatic efforts to end the conflict in Gaza amid growing concern at the rising civilian death toll.
At a joint news conference in Jerusalem with Israeli prime minister Benjamin Netanyahu, Mr Hammond put the blame for the latest outbreak of fighting firmly on Hamas while reiterating Britain's support for Israel's right to defend itself.
He said: "Britain has also been very clear that Israel has the right to defend itself and its citizens, but we are gravely concerned by the ongoing heavy level of civilian casualties. We want to see a ceasefire quickly agreed."