Sir Keir Starmer has called for Israel and Hezbollah to agree to a ceasefire, saying "escalation serves no one".
Speaking at the United Nations General Assembly, the prime minister implored the two sides to "step back from the brink".
It comes after Israeli Prime Minister Benjamin Netanyahu said his nation's troops would keep "fighting at full force" and that news of a potential ceasefire deal was "not true".
Addressing the nations of the world in New York, Sir Keir said: "I call on Israel and Hezbollah: Stop the violence, step back from the brink."
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The UK prime minister added: "We need to see an immediate ceasefire to provide space for a diplomatic settlement, and we are working with all partners to that end.
"Because further escalation serves no one."
In his speech, Sir Keir said the UN needs to "work together for peace, progress and equality" - including "preventing a regional war in the Middle East".
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Tap hereMore criticism of Russia
Earlier in his trip to the UN, Sir Keir criticised Russia for invading Ukraine in a meeting of the security council.
Both the UK and Russia are permanent members of the UN Security Council.
Addressing the general assembly, Sir Keir hinted at wanting to take further action against Russia for its invasion.
He said: "We must put new energy and creativity, into conflict resolution and conflict prevention, reverse the trend towards ever-greater violence, make the institutions of peace fit for purpose, and hold members to their commitments under the UN Charter."
Addressing the security council previously, Sir Keir said: "The greatest violation of the charter in a generation has been committed by one of this council's permanent members."
Pension to be invested in climate change
During his speech, the prime minister repeated many of his previous pledges on UK domestic policy.
However, he did appear to make one new announcement.
Sir Keir pledged to put "billions" from pension and insurance funds into "fighting climate change" and "boosting development".
The Labour government has repeatedly said it wants to use more private investment to build growth.
Sir Keir added that the plans for pensions and insurance funds would be managed through a new facility at British International Investment - part of the Foreign Office - working alongside the City of London.
The prime minister said "innovating", "thinking differently" and "moving faster" were the sort of approaches needed to deliver benefits into the future.
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Just before Sir Keir began speaking, it was confirmed he would later be meeting Donald Trump ahead of the US election in November.
However, it seems unlikely that he will be able to meet with Democrat Kamala Harris on the current visit.
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