KYIV, Ukraine—Ukrainian forces are seeking to roll back Russian gains as Moscow shifts its focus to controlling a swath of the country’s south and east, but Western officials see signs that Russia is consolidating its position to regain the offensive.

Ukrainian forces said Sunday they drove Russian troops out of Trostyanets, in the northeast near the Russian border, potentially opening a road to the provincial capital of Sumy, which is encircled by the Russians.

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