Weeks into Ukraine’s highly anticipated counteroffensive, Western officials describe increasingly “sobering” assessments about Ukrainian forces’ ability to retake significant territory, four senior US and western officials briefed on the latest intelligence told CNN.

“They’re still going to see, for the next couple of weeks, if there is a chance of making some progress. But for them to really make progress that would change the balance of this conflict, I think, it’s extremely, highly unlikely,” a senior western diplomat told CNN.

“Our briefings are sobering. We’re reminded of the challenges they face,” said Rep. Mike Quigley, an Illinois Democrat who recently returned from meetings in Europe with US commanders training Ukrainian armored forces. “This is the most difficult time of the war.”

The primary challenge for Ukrainian forces is the continued difficulty of breaking through Russia’s multi-layered defensive lines in the eastern and southern parts of the country, which are marked by tens of thousands of mines and vast networks of trenches. Ukrainian forces have incurred staggering losses there, leading Ukrainian commanders to hold back some units to regroup and reduce casualties.

“Russians have a number of defensive lines and they [Ukrainian forces] haven’t really gone through the first line,” said a senior Western diplomat. “Even if they would keep on fighting for the next several weeks, if they haven’t been able to make more breakthroughs throughout these last seven, eight weeks, what is the likelihood that they will suddenly, with more depleted forces, make them? Because the conditions are so hard.”

A senior US official said the US recognizes the difficulties Ukrainian forces are facing, though retains hope for renewed progress.

“We all recognize this is going harder and slower than anyone would like – including the Ukrainians – but we still believe there’s time and space for them to be able make progress,” this official said.

Multiple officials said the approach of fall, when weather and fighting conditions are expected to worsen, gives Ukrainian forces a limited window to push forward.

In addition, Western officials say the slow progress has exposed the difficulty of transforming Ukrainian forces into combined mechanized fighting units, sometimes with as few as eight weeks of training on western-supplied tanks and other new weapons systems. The lack of progress on the ground is one reason Ukrainian forces have been striking more often inside Russian territory “to try and show Russian vulnerability,” said a senior US military official.

Ukraine’s armed forces chief, Gen. Valerii Zaluzhnyi, told US Chairman of the Joint Chiefs of Staff Gen. Mark Milley that Ukrainian forces are step by step creating conditions for advancing. Zaluzhnyi added that he had told Milley that Ukraine’s defenses were steadfast.

“Our soldiers are doing their best. The enemy is conducting active assault actions in a number of directions, but is not succeeding,” Zaluzhnyi told Milley, according to a read out issued by the Ukrainian government.