Tony Blair has said the "tragedy" in Qana shows the situation in the Middle East "simply cannot continue".
The prime minister said there was basis for agreement to allow a UN resolution to stop hostilities on both sides, adding "we have to get it now".
Mr Blair said he was making urgent calls to world leaders, including Lebanese Prime Minister Fouad Siniora.
His comments followed the air strike on Qana in south Lebanon where more than 54 civilians, mostly children, died.
Mr Blair said "there is no basis for hostilities to continue", describing the Qana destruction as an "absolutely tragic situation".
"We have to speed this entire process up, get a resolution now, and on the passing an agreement of that resolution, then the hostilities have got to stop, and stop on all sides," he said.
However, he stressed that any solution should be long term rather than a quick fix.
"We have got to make sure that the discussions that we're having, and the negotiations that we're conducting, does lead to a genuine cessation of hostilities, in a way that allows to put an end to them for good," he said.
The BBC's political editor Nick Robinson, who is travelling with Mr Blair, said the prime minister accepted that Qana had "changed things".
Our correspondent said Mr Blair was speaking to US president George Bush as well as other world leaders in the hope of developing a package which could be voted on and agreed by the UN, leading to a resolution allowing both sides to end the conflict.
The British Ambassador to the UN, Sir Emyr Jones-Parry, said ahead of the emergency Security Council meeting, that the council ought to adopt a resolution calling for an immediate ceasefire.
"In our view - the view of the United Kingdom - there is no reason such a resolution should not be introduced into the council very quickly, and adopted as a matter of urgency," he said.
He said the UK would "work unstintingly to that end".
Earlier senior ministers spoke out in support of the prime minister after suggestions of a Cabinet split over the crisis.
They rallied around after Commons leader Jack Straw criticised Israel's "disproportionate" attacks on Lebanon.
On a four-day visit to California, Mr Blair is promoting his climate change plans and UK hi-tech business.
The prime minister is also set to to address a meeting of executives from publisher Rupert Murdoch's News Corp on the theme of leadership in the modern world.