Gordon Brown called today for an immediate ceasefire to halt the violence in Gaza.
The Prime Minister said the ongoing hostilities were a matter of "grave concern" and insisted that international pressure was being placed on both Israel and Hamas.
Last night thousands of Israeli troops poured over the border into Gaza, beginning a long-awaited ground offensive after a week of intense aerial bombardment.
Palls of thick black smoke billowed over Gaza City at first light this morning as the streets echoed to the rattling sound of machine gun fire.
Mr Brown said it was vital that the international community including the Arab League worked together to find a workable solution to the problem.
He said: "This is a very dangerous moment, I think everybody around the world is expressing grave concern.
"What we've got to do almost immediately is to work harder than we've done for an immediate ceasefire."
More than 400 Gazans have been killed and some 1,700 have been wounded since Israel began its aerial campaign, Gaza health officials said.
The UN said the death toll in Gaza included more than 60 civilians, 34 of them children.
Three Israeli civilians and one soldier have also died in rocket attacks which have reached deeper into Israel than ever before, bringing one eighth of Israel's population within rocket range.
The offensive was launched after more than a week of Palestinian rocket fire that followed a six-month truce.
The Prime Minister said assurances needed to be given to both the Israelis and Hamas.
He explained: "I can see the Gaza issues for the Palestinians - that they need humanitarian aid - but the Israelis must have some assurance that there are no rocket attacks coming into Israel.
"So first we need an immediate ceasefire, and that includes a stopping of the rockets into Israel. Secondly, we need some resolution of the problem over arms trafficking into Gaza and, thirdly, we need the borders and the crossings open and that will need some international solution."
Speaking on BBC One's Andrew Marr Show, Mr Brown said he had spoken to Israeli Prime Minister Ehud Olmert three times in the last few days to gauge what guarantees were needed for their military campaign to stop.
He said the Arab powers had to apply pressure to ensure that the illegal tunnels used for supplying Gaza with arms were closed.
The Prime Minister explained: "I sense that the Arab powers are as worried as we are about the turn of events.
"What I believe has got to happen over the next few days is that we have got to work with them and of course with the United States and the European Union, and (French) President (Nicolas) Sarkozy is visiting the region tomorrow, so that we can get both the ceasefire - and that means the stopping of rocket attacks into Gaza as well - we should get an agreement on arms trafficking and we should get an agreement on the crossings."