Yesterday's 2am strikes were launched hours before US and Ukrainian officials were due to meet in Saudi Arabia today.
President Volodymyr Zelensky was to meet Crown Prince Mohammed bin Salman in Jeddah ahead of talks involving US Secretary of State Marco Rubio.
Starmer said he hoped for a positive outcome", as he also pressed US president Donald Trump over his suspension of intelligence-sharing and military aid to Kyiv.
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No10 summarised a call between the leaders, saying: "The PM said that UK officials had been speaking to Ukraine officials over the weekend and they remain committed to a lasting peace."
Yesterday, Ukrainian drone strikes smashed Russia's Novokuybyshevsk oil refinery, 600 miles from the border, which produces fuel for Kremlin Su-27 fast jets and Tu-22M3 bombers.
Oil depots were also hit in Lipetz and Ryazan, and recruits with the 65th Mechanized Brigade of Ukrainian Armed Forces continued to train in the Zaporizhzhia region.
Zelensky stressed that there had been hundreds of attacks by Russia in the past week. Last Friday alone, 21 people were killed.
But world leaders are rallying to try to negotiate a peace deal, despite Trump taunting Zelensky - who met deputy Mecca governor Prince Badr bin Sultan bin Abdulaziz yesterday.
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Last week, Zelensky said he was "ready to work" under Trump's leadership.
Britain has been advising Ukraine on handling the peace talks, and if a deal is finally reached, the PM will lead the call with 20 allies...