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. White House National Security Council coordinator for strategic communications John Kirby addressed the speed of Ukraine’s counteroffensive Tuesday, telling CNN’s Wolf Blitzer on “The Situation Room” that “even the Ukrainians … including President Zelensky, have said that they’re not going as far as or as fast as he would like.” “While they are making progress, and they are, it’s incremental and it’s slow and it’s not without its difficulties – but they keep trying, they’re still at it,” Kirby said. “There is active fighting along that front, they are definitely trying to push forward. How far they’ll get, where that will be, what kind of breakthrough they might be able to achieve, I don’t think anybody can say right now. We’ve got to make sure we’re staying behind them and supporting them.” 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. Western officials also 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. Talking about the situation in the south, where Ukrainian forces have struggled to gain ground, Zaluzhnyi said, “Heavy fighting continues, Ukrainian troops step by step continue to create conditions for advancing. The initiative is on our side.” These latest assessments represent a marked change from the optimism at the start of the counteroffensive. These officials say those expectations were “unrealistic” and are now contributing to pressure on Ukraine from some in the West to begin peace negotiations, including considering the possibility of territorial concessions. “Putin is waiting for this. He can sacrifice bodies and buy time,” Quigley said. Some officials fear the widening gap between expectations and results will spark a “blame game” among Ukrainian officials and their western supporters, which may create divisions within the alliance which has remained largely intact nearly two years into the war. “The problem, of course, here is the prospect of the blame game that the Ukrainians would then blame it on us,” said a senior western diplomat. Last month at the Aspen Security Forum, Ukrainian President Volodymyr Zelensky pointed to the slow arrival of more advanced weapons systems from the West as reason for Ukrainian forces’ slow progress so far. “We did plan to start [the counteroffensive] in spring, but we didn’t,” Zelensky said. “Because frankly, we have not enough munitions, and armaments, and not enough properly trained brigades. I mean properly trained in these weapons.” This story has been updated with additional information.