UK Prime Minister Gordon Brown has renewed his call for an urgent ceasefire as Israeli troops and Hamas clash in the Gaza Strip.
Mr Brown also used an interview on the BBC's Andrew Marr Show to urge Arab nations to help stop the conflict.
Mr Brown's call came as it emerged that Israeli ground troops had entered Gaza after more than a week of violence.
On Saturday thousands demonstrated against the military action. Israel says it is protecting its citizens.
The Israeli government has said it is defending its people from Palestinian rocket attacks.
In the interview with Andrew Marr, the prime minister said: "What we have to to do almost immediately is work harder than we have done for an immediate ceasefire."
He repeated that this included Hamas ending its rocket attacks against Israel.
On Sunday, Israeli President Shimon Peres rejected calls for a ceasefire, while US Vice-President Dick Cheney defended the ground assault against Hamas which began on Saturday.
Mr Brown said any solution would have to include stopping the supply of arms into the region and ensuring international monitoring.
I sense that the Arab powers are as worried as we are about the turn of events
He said he had spoken to the Israeli prime minister on three occasions in the past few days and had attempted to talk to Palestinian President Mahmoud Abbas and others in the region about what could be done to stop the violence immediately.
"The blame game can continue afterwards.
"But this dangerous moment, I think, requires us to act.
"There are talks that are going on that would take us beyond the immediate violence into the sort of solutions we want but the very events we see emphasise what the real challenge is - Israel needs to be secure, Palestine needs to be viable."
Mr Brown said Arab powers had to apply pressure to ensure that illegal tunnels used for supplying Gaza with arms were closed.
He said: "I sense that the Arab powers are as worried as we are about the turn of events."
He said that during the next few days deals had to be agreed between the powers, the UK, the US and the EU which would result in a ceasefire.
"We should get an agreement on arms trafficking and we should get an agreement on the crossings," he said.
By Sunday night, Israeli ground troops and heavy armour had moved deeper into Gaza, taking up positions on either side of Gaza City and along a major east-west road - in effect cutting the territory in two.
Nevertheless, Mr Brown insisted pressure was being placed on Israel to end the violence and that the country understood this.
Earlier UK Foreign Secretary David Miliband said "intensive" diplomatic efforts to find a solution were continuing as the crisis affected the "whole world".
This appalling conflict will entrench both sides far deeper in their hostility and is exactly the wrong way to obtain the peace both Israelis and Palestinians need
Meanwhile a spokesman for former UK Prime Minister Tony Blair told the BBC the Middle East peace envoy is in Jerusalem.
He said: "He's been working on the issue from the start.
"As Quartet Representative, Mr Blair continues to be engaged on this current situation."
He said the former prime minister is due to meet Palestinian Prime Minister Salam Fayyad, Israeli Prime Minister Ehud Olmert and Defence Minister Ehud Barak and had already spoken to other international leaders.
Backbench Labour MP, John McDonnell, has demanded Parliament return early from its recess to discuss the escalating situation in Gaza.
Calling for "decisive action", he said: "We are witnessing a bloody massacre in Gaza and yet the UK government has stood by and simply repeated the usual ritual, ineffective statements of condemnation."
Shadow foreign minister David Lidington said the ground invasion was "a serious development that is bound to lead to yet more loss of life".
"The rocket attacks by Hamas on Israeli cities are acts of terrorism and must cease if there is to be a chance of restoring peace.
The stand-off near the Israeli embassy
Liberal Democrat foreign affairs spokesman Ed Davey called Israel's incursion into Gaza "a dangerous development" which would not lead to a ceasefire anytime soon.
"This appalling conflict will entrench both sides far deeper in their hostility and is exactly the wrong way to obtain the peace both Israelis and Palestinians need.
"Britain and the EU in particular must make it clear to both sides that their failure to agree a ceasefire could jeopardise the support and co-operation they will need in the future."
In the UK, demonstrators centred their attention on the Israeli embassy in London with up to 5,000 involved in a stand-off with police. Fifteen people were arrested.
The Palestinian health ministry says more than 500 Palestinians, mostly civilians, have been killed and 2,500 wounded since the assault began on Gaza.
Four Israelis - three civilians and one soldier - have been killed. The Israeli military insists about 80% of the Palestinians killed were Hamas members.
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