Theresa May vowed to support Ukraine in the face of the “threat” from Moscow as she welcomes Petro Poroshenko to Downing Street.
She told the Ukrainian president the UK stood “ready to support” his country.
Mrs May said she would continue to press for sanctions to remain in place against Russia until the Minsk agreement on a political settlement was fully implemented.
She said: “We recognise of course the threat from Russia not only seen in the annexation of Crimea but also in activity in eastern Ukraine.
“We continue to stand ready to support you against this threat, both through our bilateral security and defence co-operation but also in other ways.
“We will continue to press for sanctions to remain until the whole Minsk agreement is fully implemented.”
At the start of talks in Number 10 Mr Poroshenko thanked Mrs May for the “very firm” support for Ukraine’s sovereignty and territorial integrity “in the face of the illegal annexation of the Crimea and Russian aggression”.
He invited Mrs May to visit Ukraine but acknowledged “we know that now you have a busy schedule” – an apparent reference to the Prime Minister’s decision to call a snap general election in June.
They met as the International Court of Justice rejected Ukraine’s request for measures aimed at blocking Russian support for rebels in eastern Ukraine, but imposed measures to rein in discrimination by Moscow against ethnic Tatars and other minorities in Crimea.
The UN court said it expects both Moscow and Kiev to work to implement the Minsk agreements that were designed to bring peace to conflict-ravaged eastern Ukraine.