New Delhi, Feb 27 (IANS): The Ukraine conflict could last "a number of years" and the UK needs to be "prepared for a very long haul" in facing down Russian aggression, UK Foreign Secretary Liz Truss said, Sky News reported.

Truss praised the "very, very strong and brave" Ukrainian resistance against Russian forces moving through their country as the invasion continues into fourth day.

"This could be a number of years because what we do know is Russia has strong forces," she said.

"But we know that the Ukrainians are brave, they're determined to stand up for their sovereignty and territorial integrity, and they're determined to fight", Sky News reported.

The foreign secretary warned that Western sanctions on Moscow "will take time to have an effect and debilitate the Russian economy".

"This is not going to be, I fear, over quickly," she added. "We need to be prepared for a very long haul", Sky News reported.

In an intelligence update on Sunday morning, the UK Ministry of Defence said Russian forces were continuing to advance into Ukraine via "multiple" routes, but were still being met with "stiff resistance" from the Ukrainian military.

The UK MoD said there had been fighting within Ukrainian capital Kiev for a second night in a row but "at a lower intensity that the previous evening", the report said.

They added that Russian forces were now bypassing Chernihiv, a city 150 km to the north west of Kiev, after "encountering strong resistance" and in order to "prioritise the encirclement and isolation" of Ukraine's capital, Sky News reported.

There has also been "intensive exchanges of rocket artillery" overnight followed by "heavy fighting" in Ukraine's second city Kharkiv, the MoD said.