The war between Russia and Ukraine is “about to enter a new phase,” British intelligence officials said Saturday.

UK Defense Intelligence said in an intelligence report that Russian forces were “certainly amassing” in Southern Ukraine and that strategic fighting would now shift from the Donbas region in Eastern Ukraine to the area between the Mykolaiv Oblast, Kherson, and the Crimean Peninsula, in the South. 

“Russia’s war with Ukraine is about to enter a new phase, with the heaviest fighting shifting to a roughly 350km front line stretching southwest from near Zaporizhzhya to Kherson, paralleling the Dnieper River,” UK’s Defense Ministry said.

“Russian forces are almost certainly massing in the south in anticipation of Ukraine’s counter-offensive or in preparation for a possible assault,” the intelligence report added.

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The UK Defense Ministry also detailed a major movement of Russian troops, tanks and support vehicles into the southern region.

“Long convoys of Russian military trucks, tanks, towed artillery and other weapons continue to move away from Ukraine’s Donbas region and are headed southwest. Equipment was also reported to be moving from Russian-occupied Melitopol, Berdiansk, Mariupol and from mainland Russia via the Kerch Bridge into Crimea,” the intelligence update said.

“Battalion tactical groups (BTG), which compromise between 800 and 1,000 troops, have been deployed to Crimea and would almost certainly be used to support Russian troops in the Kherson region,” the report added, noting another BTG would likely be joining these forces “in the coming days.”

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After more than 160 days of fighting in a war that many experts thought would have lasted less than two weeks, Ukraine has successfully launched counterattacks and regained territories on its eastern front that were once occupied by Russia.

The country has also found success targeting critical transportation infrastructure that Russia was using to assist forces on its front lines.

“Ukrainian forces are focusing their targeting on bridges, ammunition depots, and rail links with growing frequency in Ukraine’s southern regions including the strategically important railroad spur that links Kherson to Russian-occupied Crimea, almost certainly using a combination of block, damage, degrade, deny, destroy, and disrupt effects to try to affect Russia’s ability to logistically resupply,” the report said.

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The report comes as Ukrainian authorities shifted and responded to a Russian offensive in Kherson last month.