US concerned for Ukraine's Kursk assault as Russia prepares counterattack

Ukrainian forces have yet to set up defensive lines as they continue their operation into the Kursk region of Russia , a U.S. official told ABC News on Wednesday.

While this might reflect Ukrainian confidence in further success for the offensive, there is concern among some American officials that failure to dig in soon could leave its troops vulnerable to a coming Russian counterattack.

"Russia didn't take it very serious at first," the U.S. official said. On Tuesday, Pentagon spokesman Maj. Gen. Pat Ryder said the U.S. had seen only a "small number" of Russian forces heading to Kursk.

But the U.S. now sees a significant second wave of Russian troops preparing to reinforce the region, coming from positions in both Ukraine and Russia, according to the official, who said some units could arrive within days, with the majority of reinforcements expected within two weeks.

It could be a costly tradeoff for Ukraine to seek incremental gains in the region at the expense of shoring up its defenses, according to Mark Cancian, former Marine colonel and senior adviser with the Center for Strategic and International Studies.

"They should draw the most defensible line inside this enclave and dig in ... and then try to hold that," Cancian said.

This advice is in line with Ukrainian President Volodymyr Zelenskyy's stated goal of creating a "buffer zone" inside Russia.

"When you're on the attack, you tend to take more casualties," Cancian said. "And it would be fine if that opens up the front for some follow-on movement, but that's doesn't appear to be what's going on. It looks like they're just sort of plodding forward."

Despite the danger posed by incoming Russian forces, and risks of being overextended, having foreword units cut off, or leaving other areas of the front undermanned, experts say Ukraine's initiative in Kursk has already succeeded in forcing Russia to make hard decisions about how to allocate its finite resources; in boosting confidence in the Ukrainian military both domestically and with key allies; and in obtaining territory that could be used as bargaining leverage later on.

The Kremlin was by all accounts taken off guard by Ukraine's incursion, but Kyiv might itself have been surprised by its quick gains.

"It was initially intended for psychological purposes, similar to the Doolittle Raid after Pearl Harbor, but it has evolved based on its success," said Mick Mulroy, an ABC News contributor who served as a CIA paramilitary officer and deputy assistant secretary of defense.

Ukrainian forces have now been in the Kursk region for more than two weeks.

"Over the next couple days, we'll see what the Ukrainians do and whether they keep this strategy of just nibbling away, whether they go on to the defensive, whether they try to make a big attack, which I think is unlikely, but not impossible," Cancian said.