Tony Blair has said a plan is being drawn up to secure an "immediate cessation of hostilities" in Lebanon.
But the prime minister said it will take a "few days" to finalise because both sides had to make concessions.
"It will not stop on both sides unless there is a plan to make it stop and that's what we are working on urgently, as we speak," said Mr Blair.
He said what was happening in Lebanon was a "catastrophe" but innocent Israeli civilians had been killed too.
"I don't want the killing to go on. I want the killing to stop now.
"But it has got to happen on both sides. It is not going to happen on both sides, without a plan to make it happen," he told a news conference Downing Street.
He rejected suggestions the UK and US did not want an immediate ceasefire - and that they had taken sides with Israel.
He said "anybody with any sense or humanity" wanted the conflict to end.
"We are fully aware of what is happening to the reputation of the West, of relations between the Arab and Muslim world, as this continues," he told reporters.
"But if it is to stop it's going to have to stop on both sides and that's not going to happen unless you have a plan to make it happen.
"And I can assure you that we are bending every single bit of our diplomatic, our political effort to make sure that happens as swiftly as possible."
Speaking alongside the Iraqi prime minister, Nuri al-Maliki, Mr Blair said the underlying causes of the conflict also had to be addressed, in the longer term.
And he called for an international force to keep the peace in southern Lebanon.
"The plan is precisely to make sure that you put in place the conditions - which will obviously mean not just the cessation of hostilities but the release of the kidnapped soldiers - and agreement on both sides that hostilities have got to stop.
"And then I believe we will need some form of international force in the South, in Lebanon, which we must build up over time to make sure that that can then act as a buffer between those people on the Lebanese side who want to cause difficulty, and Israel."
He said he had been working on the plan with the Americans and Britain's EU partners since last week's G8 conference.
Mr Nouri al-Maliki warned violence in Lebanon could lead to further extremism in the region.
"What is going on in Lebanon will be a great push toward fundamentalism," he told reporters.
And he added: "What is going on in Lebanon and what we in Iraq see on television and other Iraqis see on television will backfire on the stability process in Iraq."
His comments came as the leader of the UK's third largest party, the Liberal Democrats, Sir Menzies Campbell, called on Mr Blair to suspend arms exports from the UK to Israel "in light of disproportionate military action by Israel in Lebanon and Gaza".
Mr Blair declined to comment on Sir Menzies' call.
US Secretary of State Condoleezza Rice earlier held talks with Lebanon's PM in Beirut at the start of a Middle East tour to discuss the regional crisis.
En route from Washington for the unannounced visit, Ms Rice said there was an "urgent" need for a ceasefire - but that conditions had to be right.
The UK government has meanwhile announced a further £2.2m to the Lebanon humanitarian relief effort, bringing the total UK contribution to £5m.