Liz Truss, the UK foreign secretary, said the Ukraine conflict could last for a long time and the UK needs to prepare for prolonged combat against Russian aggression.
Despite the invasion continuing into its fourth day, Truss commended Ukraine for their "very, very strong and brave" resistance.
"We do know that Russia has strong forces, so this may take a number of years," she said.
We know that the Ukrainians are courageous and determined to defend their independence and territorial integrity, and they are ready to take the fight to the enemy", Sky News reported.
Western sanctions against Moscow will take some time to make an impact, and they will harm the Russian economy in the long run, suggests the foreign secretary.
I believe this won't end quickly, she said. "We need to be prepared for a very long haul," Sky News reported.
As stated in a Sunday morning intelligence update, Russian forces continue to advance into Ukraine via "multiple" routes against "stiff resistance" from the Ukrainians.
A report from the UK MoD says fighting erupted in Kiev for the second night in a row, albeit "at a lower intensity" than the previous evening.
The sources also noted that the Russians bypassed Chernihiv, 150 kilometres away from Kiev, since "strong resistance" was encountered there, and they wanted "to focus on isolating and encircling Kiev.".
In Kharkiv, Ukraine's second city, "intensive rocket artillery exchanges" followed by "heavy fighting" occurred overnight, the MoD reported.