Ukraine is kicking off its big offensive armed with powerful Western tanks and weapons, but breaking Russian lines may be anything but easy

• Ukraine's much-anticipated counteroffensive has finally begun.

• Ukrainian forces are armed with Western-made weapons, but they are facing tough Russian defenses.

• The initial phases of this operation may be the most dangerous and costly, war experts said.

The Ukrainian counteroffensive has begun, kicking off a particularly dangerous time for Ukraine's armed forces, even with its newly acquired arsenal of Western-made tanks, armored vehicles, and other weaponry that has been pouring in over months.

"Amidst a highly complex operational picture, heavy fighting continues along multiple sectors of the front," the British defense ministry said in a Thursday update, adding that "in most areas Ukraine holds the initiative."

Ukrainian forces, including combat units trained in Western tactics and armed with Western weaponry, are pushing against various parts of the front line, The Washington Post reported. The Financial Times reported that Ukraine had committed Western-made tanks, which experts have said would signal the start of the offensive.

Kyiv hasn't announced it, as it said it wouldn't, but Ukrainian officials told ABC News that the counteroffensive was underway. An officer and a soldier told NBC News the same, and the Institute for the Study of War, a think tank that has carefully tracked Russia's war in Ukraine, said on Thursday that "the Ukrainian counteroffensive has begun."

There appear to be Ukrainian offensive operations against Russian defenses in the Zaporizhzhia and Donetsk oblasts, among other locations along the front lines. Ukraine has seemingly made some gains, but other attacks appear to have been less effective.

The fog of war makes determining a main line of effort difficult, but one objective of the counteroffensive, given Ukrainian activities, may be severing the so-called land bridge connecting Russia and Russian-controlled territories in the southeast to cut critical supply lines for Russian forces.

Regardless of the objective, Ukrainian forces are most likely facing a tough fight, and failure to make meaningful progress could risk threatening the aid that has fueled Kyiv's war effort and even push Ukraine to negotiate. As some experts have said, the stakes for the Ukrainian offensive are high.

Partner nations provided Ukraine with mountains of military hardware, including formidable third-generation German-made main battle tanks, armored infantry fighting vehicles, and massive amounts of ammunition ahead of its counteroffensive punch.

Ukraine also received training in Western tactics and best practices for employing the new additions to the Ukrainian arsenal.

US Army Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, told CNN this week that Ukraine's armed forces were "very well prepared" for the counteroffensive. However, he added that it's "too early to tell what outcomes are going to happen. Offense is hard and fighting tends to favor the defenders.

Russia has spent months digging extensive trench networks, building barriers, and reinforcing its lines in preparation for Ukraine's much-anticipated offensive. Unlike last summer's sweeping offensive around Kharkiv, Ukrainian forces face much more heavily defended positions.

The New York Times reported earlier this week that capturing a trench was considered "difficult soldiering" that required careful planning, coordination of combat assets, and significant amounts of ammunition and firepower. In a push to capture an enemy trench in May, assault forces fired 3,000 rounds from just two machine guns.

"The initial counteroffensive operations may be the most difficult and slowest, as they involve penetrating prepared defensive positions," ISW said Thursday. "Initial setbacks are to be expected. This phase may also see the highest Ukrainian losses."

Citing two senior US officials, CNN reported Thursday that Ukrainian forces had met greater resistance than expected and had suffered heavy equipment and personnel losses trying to break through Russian lines in the eastern part of the country.

One official said the losses, which included armored vehicles, were "significant."

The Russian Ministry of Defense has released a video, which Insider was not able to independently verify, that purportedly shows the loss of a German-made Leopard tank in a battle near Novopokrovka. Russia was recently mocked for presenting a strike on farm equipment as a tank kill, but the newest video does appear to show a loss, according to open-source intelligence channels following the war in Ukraine.

But losses, within limits, are an expected part of combat, especially high-intensity warfare like that in Ukraine, where both sides have suffered horrific numbers of casualties.

"We strongly believe that we will succeed," Ukrainian President Volodymyr Zelenskyy told The Wall Street Journal last weekend, adding that he also expected "a large number of soldiers will die."

ISW said Thursday that "militaries have long identified the penetration phase of a mechanized offensive as the most dangerous and costly."

The think tank added, as Russia makes claims of thwarting Ukraine's offensive, that "the success or failure of this phase may not be apparent for some time."

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