Prime Minister David Cameron gave Benjamin Netanyahu his "strong support" for Israel's right to take "proportionate action" to defend itself.
Mr Cameron spoke to his Israeli counterpart as the death toll mounted on both sides in Gaza and Downing Street said the Prime Minister expressed condolences for the loss of Israeli troops while raising concerns about the civilian casualties.
The phone conversation came as 65 Palestinians and 13 Israeli soldiers were killed in the first major ground battle of the offensive.
The United Nations Security Council was holding an emergency meeting on the situation in Gaza early today.
A Downing Street spokesman said the two leaders agreed that the only way to end the "spiral of violence" was a lasting ceasefire observed by both sides.
He said: "T he Prime Minister noted the ongoing rocket attacks from Hamas and reiterated the UK's strong support for Israel's right to take proportionate action to defend itself from them.
"The Prime Minister expressed condolences for the deaths of Israeli troops and concern about mounting civilian casualties in Gaza.
"The Prime Minister agreed with Prime Minister Netanyahu that the way out of the spiral of violence was for both sides to observe a lasting ceasefire in line with the initiative that was put forward by Egypt."
Mr Netanyahu has the offensive will continue "as long as necessary" to end attacks from Gaza on Israeli civilians.
Earlier, Foreign Secretary Philip Hammond said "we will be looking very carefully" to make sure that civilians are being protected as Israeli troops continued their offensive.
He said there needed to be a ceasefire and longer-term solution to the situation to prevent it flaring up every few years.
On BBC1's Andrew Marr Show he said: "I srael has a right to defend itself against these rocket attacks coming from Gaza, but it must do so in a way that is proportionate and it must take all measures necessary to prevent unnecessary loss of civilian life."
Asked three times whether Israel's military operation had been a proportionate response, Mr Hammond said: " I have asked the Israelis to use every effort they can to minimise the loss of civilian life.
"Now we will be looking very carefully at what is happening on the ground to make sure that it is proportionate, that civilian lives are being protected in a proper way.
"But Israel does have the right to protect itself and the best way to avoid Palestinian loss of life in Gaza is for Hamas to stop firing rockets from Gaza and in those circumstances the world would not expect any kind of Israeli action against Gaza."
Mr Hammond said the international community was trying to "stop the bloodshed, reimpose a ceasefire and then find a longer-term solution".
He said: "We can't deal with Gaza by having a flare-up every two years - hundreds of people killed, another ceasefire which then breaks down 18 months, two years later. We have to get to the underlying causes.
"We have to re-establish the role of the Palestinian authority and the governance of Gaza and get some order out of this chaos."
On Sky News' Murnaghan programme he added: "It's always the women, the children and the elderly who suffer the most and our hearts go out to the Palestinian people trapped in Gaza who are suffering appalling losses.
"But look, the fact is Israel has a right to protect itself against attack and Hamas has been using those areas of Gaza to launch rocket attacks systematically against Israel."
Last week Deputy Prime Minister Nick Clegg said I srael's bombardment of the Gaza Strip amounted to a "deliberately disproportionate form of collective punishment".