LONDON (Reuters) – The rate of Russian air and artillery strikes in Ukraine over the past 24 hours has been lower than in previous days but Russian forces were believed to be advancing in the south of the country, Britain said in an intelligence update. “Ukraine continues to hold the key cities of Kharkiv, Chernihiv and Mariupol,” the British defence ministry said on Twitter early on Saturday. “There have been reports of street fighting in Sumy. It is highly likely that all four cities are encircled by Russian forces.” The ministry said Russian forces were probably advancing on the southern po…