KYIV, Oct 26 (AFP) - A senior Ukrainian official predicted "the heaviest of battles" to come for the partially Russian occupied strategic southern province of Kherson and said Moscow's military is digging in to face advancing Ukrainian forces.

The region's capital city and river port Kherson, which had a pre-war population of about 280,000, is the largest urban centre Russia still holds since capturing it early in the invasion of Ukraine eight months ago.

Ukrainian forces do not appear to have gained much ground in their counter-offensive in Kherson since early October, when Russia claimed to have annexed the province and three others, a move condemned by 143 countries at the United Nations as an "attempted illegal annexation".

"With Kherson everything is clear. The Russians are replenishing, strengthening their grouping there," Oleksiy Arestovych, adviser to Ukrainian President Volodymyr Zelenskiy, said in an online video on Tuesday evening.

"It means that nobody is preparing to withdraw. On the contrary, the heaviest of battles is going to take place for Kherson," according to Arestovych, who did not say when the battle might happen.

Of the four provinces Russian President Vladimir Putin proclaimed to have annexed, Kherson is arguably the most strategically important. It controls both the only land route to the Crimea peninsula Russia seized in 2014 and the mouth of the Dnipro, the vast river that bisects Ukraine.