In its early phases, Ukraine's counteroffensive is having less success and Russian forces are showing more competence than western assessments expected, two western officials and a senior US military official tell CNN.
The counteroffensive is "not meeting expectations on any front," one of the officials said.
According to the Western assessments, Russian lines of defense have been proving well-fortified, making it difficult for Ukrainian forces to breach them. In addition, Russian forces have had success bogging down Ukrainian armor with missile attacks and mines and have been deploying air power more effectively.
Ukrainian forces are proving "vulnerable" to minefields and Russian forces "competent" in their defense, one of the Western officials said.
The officials cautioned that the counteroffensive is still in its early stages -- and that the US and its allies "remain optimistic" Ukrainian forces will be able to make territorial gains over time. The US and its allies are likely to wait until at least July for a fuller assessment of the progress of the counteroffensive which was gradually launched over the last few weeks and is seen as crucial to determining who ultimately wins the war which was launched when Russia invaded last year.
In addition, these officials note that Ukrainian forces have themselves been adapting to Russian tactics and defenses, including carrying out more dismounted operations. In recent days, Ukrainian forces have also had more success targeting and shooting down Russian aircraft.
Ultimately, the counteroffensive is proving a "tough drive" for Ukraine and Russia, one of the Western official said, with both sides incurring heavy losses.
Ukrainian President Volodymyr Zelensky admitted Wednesday that progress had been "slower than desired."
"We would definitely like to make bigger steps," Zelensky acknowledged in a BBC interview. "But nevertheless, those who fight shall win and to those that knock, the door shall be opened."
Well in advance of the counteroffensive, Western officials cautioned that forces defending territory always maintain significant advantages, especially given the weeks Russian forces have had to dig in and fortify their defensive lines.
Several officials told CNN that adverse weather was proving an issue for Ukrainian forces.
"The weather has been playing havoc with the offensive schedule as vehicles have struggled with trafficability. Ukrainian casualties are heavy, though not as bad as the Russia's are trying to portray," one of the officials said.
After a meeting of the Defense Contact Group in Brussels last week, the top US general said Ukraine is making "steady progress" but warned they had to be in for the long haul.
"This is a very difficult fight, it is a very violent fight and It will likely take a considerable amount of time and at high cost," Chairman of the Joint Chiefs of Staff Gen. Mark Milley said.
Referring to Ukraine losing armored vehicles in the early days of the counteroffensive, US Defense Secretary Lloyd Austin said, "the Ukrainians have the ability to recover equipment that's been damaged, repair where possible, get that equipment back into the fight,"
"There will continue to be battle damage" but Ukrainian forces "still have a lot of combat capability, combat power," Austin added.