BBC - Ukrainian President Volodymyr Zelensky has said his chief of staff, Andriy Yermak, has resigned following an anti-corruption raid on his home.
Yermak, 54, has been Zelensky's closest adviser throughout Russia's full-scale war, but has come under increasing pressure over an escalating scandal - even though he has not been accused of any wrongdoing.
Zelensky recently appointed his chief of staff to head crucial negotiations, with US President Donald Trump leading a new drive to end the war with Russia.
In a stark address to the nation outside his presidential office, Zelensky called for unity, warning: "We risk losing everything: ourselves, Ukraine, our future."
The corruption scandal has rocked Ukraine for weeks, weakening Zelensky's own position and jeopardising the country's negotiating position with the US at a delicate time.
Ukraine's two anti-corruption agencies raided Yermak's apartment in Kyiv's government quarters early on Friday and the chief of staff said on social media that "from my side there is full co-operation".
"I'm grateful to Andriy that Ukraine's position on the negotiating track was always presented as required: it was always a patriotic position," Ukraine's president said during his video address in Kyiv on Friday.
Zelensky said he would start consultations on Saturday on who would replace Yermak as his top adviser: "When all the attention is focused on diplomacy and the defence in a war, inner strength is required."
"Russia wants Ukraine to make mistakes – there won't be any mistakes from our side. Our work continues, our fight continues. We have no right... to retreat, to quarrel [among ourselves]."
Hungary's Orban defies EU partners and meets Putin again in Moscow
Putin doubles down on demands for Ukrainian territory ahead of talks with US in Moscow
Yermak's departure from Ukraine's leading circle will come as a major blow for Zelensky, with US Army Secretary Dan Driscoll due to arrive in Kyiv by the end of this week as part of Trump's draft peace plan. US officials are heading to Moscow next week.
Even hours before the raid on his flat, Yermak underlined the significant influence he held in Ukraine's leadership, conveying his government's position as it comes under US pressure to make territorial concessions to Russia, which launched a full-scale invasion of Ukraine in February 2022.
"As long as Zelensky is president, no-one should count on us giving up territory. He will not sign away territory," Yermak told The Atlantic website.
Russia has demanded that Ukraine hand over territory it still controls in the eastern Donetsk region, including several strategically important cities.
"If they don't withdraw, we'll achieve this by force of arms," Putin said on Thursday.
Yermak admitted during his interview he was under "enormous" pressure to stand down, adding "the case is fairly loud, and there needs to be an objective and independent investigation without political influence".
Investigators have linked several high-profile figures to an alleged $100m (£75m) embezzlement scandal in the energy sector.
Ukraine's anti-corruption bureau Nabu and specialised anti-corruption prosecutor's office (Sap) said they had uncovered an extensive scheme to take kickbacks and influence state-owned companies including state nuclear energy firm Enerhoatom.
Russian officials involved in Trump's draft peace plan have talked up the corruption claims, which have alarmed European Union allies.
Ukraine is a candidate to join the EU and a report earlier this month highlighted doubts over "commitment to its anti-corruption agenda".
Zelensky has already fired two ministers and several suspects have been detained in the scandal. One of the president's former business associates, Timur Mindich, has fled the country.
Mindich was co-owner of the TV studio Kvartal 95 where Zelensky's acting career took off before he was elected president.