The focus must remain on Ukraine, Starmer told Zelenskyy when he received the Ukrainian leader on Tuesday.
Dutch Prime Minister Mark Rutte was also present at the London meeting.
The discussion included increased military cooperation between Ukrainian expertise and the British defence industry.
The UK is also pledging 500,000 pounds, equivalent to just over six million kronor, to build a centre for artificial intelligence in Kyiv.
Starmer then reminded the public of a changed world situation:
There is obviously a conflict in Iran and the wider Middle East, but we must not lose focus on what is going on in Ukraine and the need for our support there.
The British Prime Minister also noted that the US and Israel's war in Iran benefits Russia, as Russia's oil revenues increase with rising oil prices. The US has also eased sanctions on Russian oil exports.
President Zelenskyy has not commented on the meeting with Starmer or the tea with King Charles.
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