According to British intelligence, Russian forces have very likely secured the center of the fiercely fought city of Bakhmut in eastern Ukraine, threatening a crucial supply line for Ukrainian forces to the west.

When asked to comment on the story, a spokeswoman for the Ukrainian military command told Reuters that the situation in Bakhmut was challenging and that Russian forces were devoting all efforts to conquer the city but were not achieving “strategic success.”

The months-long battle for Bakhmut, one of the last urban centers in Ukraine’s eastern Donetsk province yet to fall to Russian forces, has proven one of the bloodiest of the war.

The British report, if confirmed, indicates that Ukrainian forces are under severe pressure, two days after President Volodymyr Zelenskiy said they would withdraw if at risk of encirclement.

Donetsk is one of four provinces in eastern and southern Ukraine that Russia is seeking to fully occupy. It declared them annexed last year after failing to seize the capital Kyiv in an attempt to overrun Ukraine early in the war.

Friday’s daily update from British intelligence contrasted with the usual emphasis on Ukrainian military successes.

“Russia has made further gains and has now highly likely advanced into the town centre, and has seized the west bank of the Bakhmutka River. Ukraine’s key 0506 supply route to the west of the town is likely severely threatened,” it said.

Eastern Military Command spokesperson Serhiy Cherevatyi told Reuters that Ukraine controlled the situation in Bakhmut and understood Russian intentions.

“The situation is difficult, the enemy is concentrating maximum efforts to capture Bakhmut. However it is suffering serious losses and not reaching strategic success,” Cherevatyi said.

Western analysts say both sides are losing large numbers of troops in the battle for Bakhmut.

They have played down the strategic significance of the city but Ukraine has framed its dogged defence as a way of wearing down Russian forces before a counter-offensive bolstered by advanced Western-supplied weapons.