Rishi Sunak has pledged to stand with Israel in its "darkest hour," adding he wanted the country to "win" its war with Hamas.
Speaking after a meeting with Israeli counterpart Benjamin Netanyahu, the PM said the Palestinian people were "victims of Hamas too".
He repeated the UK supported Israel's right to self-defence, in line with international law.
Mr Netanyahu said Israel needed "continual support" for a "long war".
The meeting between the two leaders is part of a two-day visit by Mr Sunak to the wider region. No 10 has confirmed he will visit Saudi Arabia later on Thursday, among trips to regional capitals.
Calling the Palestinian militant group the "new Nazis," the Israeli PM called the conflict with Hamas a "battle of the entire civilised world".
Israeli warplanes and artillery have been bombarding Gaza since more than 1,400 people were killed in the unprecedented Hamas attack on Israel on 7 October.
Gaza remains under siege, with Israel blocking cross-border supplies of water, electricity, and fuel.
Speaking to reporters after the meeting with Mr Netanyahu, Mr Sunak said the UK "absolutely" supported Israel's right to defend itself, whilst adding this must be within international law.
The UK prime minister said Israel was "taking every caution" to prevent harm to civilians - adding this was "in direct contrast" to Hamas.
He also said he appreciated Israel's announcement on Wednesday it would not stop aid entering southern Gaza from Egypt.
Israel says it will not allow any aid to pass through its own territory until hostages taken by Hamas during its attack earlier this month are released.
After an earlier meeting with Israel's president Isaac Herzog, No 10 said Mr Sunak hoped for "further progress" in delivering aid to Gaza.
Earlier, the prime minister said Israel had suffered a "horrific act of terrorism" at the hands of Hamas.
The visit comes directly after US President Biden's, as world leaders step up efforts to prevent the conflict with Hamas spilling into the wider region.
After his trip to Israel, Mr Sunak is due to meet Crown Prince Mohammed bin Salman during his trip to Saudi Arabia. Details for other legs of the trip have not yet been confirmed.
