Ukrainian troops are counter-attacking Russian forces in some areas of the country, with reports that they have gained ground near the capital, Kyiv.
Local authorities in the town of Makariv, west of Kyiv, said Ukrainian flags were flying there once more.
A US defence spokesman said Ukrainians were also reversing momentum in some parts of the south.
In the small southern town of Voznesensk, Russian forces were pushed back and an armoured convoy destroyed.
And in Kherson, close to the Crimean peninsula and the first city to fall to Russia, Ukrainian forces are also trying to recapture territory.
However, the latest assessment from the UK Ministry of Defence says Russian troops in Ukraine are moving in from the north and south to "envelop Ukrainian forces in the east of the country".
The note says "Russian forces are likely reorganising before resuming large-scale offensive operations".
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