Ukraine breached Russia's defensive line on Wednesday and was able to deploy vehicles past it.
It was a major achievement, but analysts said there is still a long way to go.
"I think ultimately it's going to boil down to reserves and availability of ammunition," one expert said.
Russia will push back hard against Ukrainian troops who breached its main defensive line with armored vehicles this week, analysts told Insider.
Ukraine took a major step forward Wednesday, seeming to push past a fierce defensive line in the southeast of the country with vehicles for the first time.
Ukraine breached the area near the village of Verbove in the southeastern Zaporizhzhia region. It was a major dent in the so-called "Surovikin Line", but there are more layers of defense to go.
Ukraine still has a hard fight ahead, experts on the war told Insider.
Marina Miron, a researcher at the Department of War Studies at King's College London, was one.
"There is about a mile to the actual village of Verbove, which means Ukrainian soldiers will have to cross open fields and face Russian artillery fire and Russian drones," she told Insider.
"So it will be very difficult for the Ukrainians. It remains to be seen whether or not they can get there and consolidate the foothold in order to push the Russians back."
Russia already deployed units to combat the counteroffensive by blasting at the flanks of the Ukrainian troops, which were taking heavy casualties, The Wall Street Journal reported Thursday.
"We are pushing through," an unnamed Ukrainian officer involved in the fighting told the outlet. "We are destroying them. But the price …"
Franz-Stefan Gady, a former senior fellow with the Institute for International Strategic Studies, told Insider that Ukraine needed to protect its flanks to stop Russia cutting off its advance.
"I think ultimately it's going to boil down to reserves and availability of ammunition," he said.
"And in that sense, we don't really have a good picture of which of the two sides is going to struggle with attrition because it remains a fight dominated by attrition," he said.
Gady was hesitant to say whether the breach would expand into a breakthrough — as was the defense analyst Michael Kofman, who also spoke to Insider.
Kofman is a senior fellow at Carnegie Endowment for International Peace, and one of the most prominent commentators on the war.
"Current progress through the second line, outside Verbove, is unclear and it is too early to claim that a breach has turned into a breakthrough," Kofman told Insider.
Despite this, Russia should still be worried, said a final analyst — Johan Huovinen at the Swedish Defense University.
"This is the first major crack in their defense … of course, the Russians should be worried," he said.
Russia, for its part, has disputed that a breakthrough occurred at all. An official cited by the state-run RIA Novosti agency said that Ukraine had tried to breach Russia's lines but was forced to retreat.
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