LONDON (Reuters) - British Prime Minister Liz Truss on Tuesday will tell other Group of Seven leaders to remain steadfast in their support of Ukraine and request an urgent meeting of NATO leaders, Downing Street said, after Russia stepped up attacks on Ukrainian cities. Russian President Vladimir Putin has mobilised more troops and issued nuclear threats following a Ukrainian counter-offensive to retake land in the east of the country. Britain has said that Russian missile strikes on civilian areas in Ukraine are unacceptable, and Truss has said the virtual G7 meeting, which Ukrainian President Volodymyr Zelenskiy will also join, is a chance to reaffirm the unity of opposition against the invasion. Nobody wants peace more than Ukraine. And for our part, we must not waver one iota in our resolve to help them win it, Truss is expected to say at the meeting, according to extracts released by her office. Russia invaded Ukraine in February, in what Moscow terms a special operation. Britain is among the countries that have directed sanctions at Russia in response. Truss will say that Ukrainian military advances had been in part facilitated by military aid and humanitarian assistance by the international community, as well as the impact of sanctions on Russia. She will say sanctions should be maintained and request a full meeting of leaders from the NATO military alliance in the coming days. The overwhelming international support for Ukraine's struggle stands in stark opposition to the isolation of Russia on the international stage, she will say. (Reporting by Alistair Smout; Editing by Tomasz Janowski)