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.