William Hague has called for immediate action to quell the crisis in Gaza during telephone calls to Palestinian president Mahmoud Abbas and Israeli foreign minister Avigdor Lieberman.
The Foreign Secretary said Israel had a right to defend itself from rocket attacks but urged both sides to work to restore a ceasefire and said Britain stands ready to provide what support it can.
Palestinians have put the death toll at more than 125 after Israel widened its air strikes on the Gaza Strip as Hamas continued to launch hundreds of rockets.
Mr Hague said: "I told minister Lieberman that continuing rocket attacks from Gaza are completely unacceptable. Israel has a right to defend itself against such attacks, but the whole world wanted to see de-escalation.
"I welcomed president Abbas's call for a ceasefire agreed by both sides, and urged him to do all he could to help bring this about. I offered my condolences for the loss of civilian lives in Gaza."
Mr Hague will talk to international counterparts, including US secretary of state John Kerry, about the need for "urgent, concerted" action to secure a ceasefire.
"The current crisis only underlines the need for a fundamental transformation of the situation in Gaza, including the restoration of Palestinian Authority control, the opening up of legitimate movement and access, and a permanent end to the unacceptable threat of rocket attacks and other forms of violence from Gaza against Israel," he added.
"It is also a stark reminder of the need to make progress toward a permanent peace, based on the two-state solution. However difficult that looks at this time, there is no alternative that can deliver peace and security for both Israelis and Palestinians.
"This is a time for bold leadership. Both sides should take the necessary steps towards a lasting peace and to address the underlying causes of the conflict and instability in Gaza. I stressed that the UK stands ready to do all it can to support this goal."
Labour's Douglas Alexander warned the spiralling violence risks "fuelling yet more extremism and conflict".
The shadow foreign secretary said: "The spiral of violence that has engulfed Gaza, southern Israel and the West Bank is bringing untold suffering to innocent people.
"I condemn the firing of rockets into Israel by Gaza-based militants. No government on earth would tolerate such attacks on its citizens but the rapidly growing number of Palestinian civilians killed in Gaza by Israeli air strikes risks fuelling yet more extremism and conflict.
"The life of a Palestinian child is worth no less than the life of an Israeli child. Every life is equal, irrespective of religion or nationality. A spiral of violence that reinforces the insecurity of Israelis and the humiliation of Palestinians leads only to further suffering.
"This is a time and this is a crisis that demands not revenge, but statesmanship motivated by justice.
"All sides have a responsibility to respect in full the November 2012 ceasefire and to address the underlying causes of conflict and instability in Gaza and the region.
"The people of Israel have the right to live without constant fear for their security, just as the people of Gaza also have the right to live in peace.
"The UK Government must now work with international allies to encourage restraint, dialogue and efforts towards peace at this time of great and grave danger."