US and western officials see signs Ukraine's counteroffensive is beginning

US and western officials see signs that Ukraine's long-awaited counteroffensive against Russia is beginning and have noted a "substantial increase in fighting" in the east of the country over the last 48 hours as Ukrainian troops probe for weaknesses in Russian defensive lines, a senior NATO official said on Tuesday.

While preliminary attacks, also known as "shaping" operations, have been underway for at least two weeks, Ukrainian forces have in the last several days begun testing Russian positions with artillery strikes and ground attacks to find vulnerable areas they can break through, the NATO official and a senior European military intelligence official told CNN.

The destruction of a sprawling dam in Ukraine's Russian-occupied Kherson region, which triggered a wave of evacuations on Tuesday as floods of water spilled from the Nova Kakhovka hydro-electric plant, could complicate some of Ukraine's plans, officials told CNN.

The dam's destruction could now make it more difficult for Ukrainian troops to cross the Dnipro River and attack Russian positions there, said two western officials. And the dam's collapse has already created a significant humanitarian challenge into which the Ukrainian government will need to address and funnel resources.

"Anything that may have been planned downstream from the dam probably has to be replanned," a European ambassador in Washington said. "Ultimately, the water levels will recede, but most likely, the catastrophic flooding has impacted the bridges and roads in the area, so they may not be usable in the way as planned before."

The US and the western intelligence community are still examining who is responsible for the dam's destruction, but officials are leaning toward Russia as the culprit, the official said.

Over the last several days, analysts have seen some notable Ukrainian operations and probes in the southeastern region of Zaporizhzhia, between the southern city of Kherson and the Zaporizhzhia nuclear power plant, the NATO official said. Ukrainian forces are also conducting operations south of Donetsk city in eastern Ukraine, which appears to be a new effort, the western official said.

The counteroffensive is expected to be carried out on multiple fronts, a senior US military official said. The official added that "there are many moving parts to synchronize" before a major ground operation can be launched. The weather has also played a role and delayed Ukraine's initial attacks on Russian defensive lines.

Ukraine's Deputy Defense Minister Hanna Maliar told Ukrainian television on Monday that an offensive is "taking place in several directions."

The European official noted that Ukrainian military will "have to find one to two gaps" where they can punch through and advance, pushing Russian troops aside before they can move in with a larger force.

It will be difficult to determine when these shaping attacks end and an offensive begins, the NATO official said, and the world should not expect Ukraine to announce that it has begun a major military operation; Ukrainian officials have repeatedly said as much.

Ukraine also has been sharing less information about its military plans with the US since hundreds of classified Pentagon documents leaked online earlier this year, including some that described Ukraine's military capabilities and operations, a defense official told CNN.

But Russia, for its part, remains "seemingly unable to launch a significant offensive of any kind," the NATO official added, giving Ukraine an advantage as troops probe for gaps in Russian defenses.

Still, Ukrainian President Volodymyr Zelensky is tempering expectations. He told The Wall Street Journal last week that he believed the counteroffensive will be successful but was not sure how long it will take, especially without advanced western fighter jets. Ukraine will soon begin training on F-16s and other fourth generation fighter aircraft in Europe, but it could be months before the aircraft is in country.

"Everyone knows perfectly well that any counteroffensive in the world without control in the skies is very dangerous," Zelensky said. "Imagine what a military man feels, knowing he does not have a 'roof' and he can't understand how neighboring countries have that."

Chairman of the Joint Chiefs of Staff Gen. Mark Milley also told CNN that he believes the Ukrainians are "very well-prepared" for the offensive. But, he added, it remains "too early to tell what outcomes are going to happen."