T-72B3 tanks of the tank force of the Russian Western Military District conduct field firing at Kadamovsky Range. Erik Romanenko/TASS
CNN  — 

Intercepted communications obtained by the US have revealed that some Russian officials have worried that a large-scale invasion of Ukraine would be costlier and more difficult than Russian President Vladimir Putin and other Kremlin leaders realize, according to four people familiar with the intelligence.

Three of the sources said those officials include intelligence and military operatives.

The officials have also grumbled about their plans being discovered and exposed publicly by western nations, two of the sources said, citing the intercepted communications.

There is no evidence that these officials are opposed to the overall plan, or would revolt against Putin’s orders, two of the sources said. Another source familiar with the US intelligence noted that Russia has a professional military that would be expected to effectively carry out Putin’s orders.

Still, Russian defense personnel believe it will be difficult, a senior European official said. “In the assessments we see it is clear some people on the [Russian] defense side are not really understanding what the game plan is,” the official said. The official added that the assessments suggest the defense personnel think “it’s a very difficult game plan to stand up.”

Another of the sources familiar with the intelligence said that the way the Russian planning has evolved and expanded over the last two months suggests that the concerns have been heard by senior Russian officials.

Officials told CNN on Saturday that Putin has now assembled 70% of the military personnel and weapons on Ukraine’s borders that he would need for a full-scale invasion of the country. Some assessments indicate that he may be planning to try to take Ukraine’s capital, Kyiv, they said, and the city could fall within 48 hours of a large-scale land and air invasion.

Another possibility, the officials said, is that Putin decides upon a multi-pronged operation, sending in forces from several directions across Ukraine to quickly fracture the Ukrainian military’s ability to fight as a cohesive force – a classic Russian military strategy.

“We’ve long said that Putin continues to add to his options and capabilities, and we’ve been equally transparent about some of the moves we believe he intends to make to justify some sort of military action,” Pentagon press secretary John Kirby told CNN Monday. “We’re also going to continue to help Ukraine better defend itself with both lethal and non-lethal assistance.”

US officials publicly and privately say they still don’t know whether Putin has decided to attack, and what form it would take if he did. That is largely because the US’ visibility into Putin and his inner circle is still extremely limited, the sources told CNN.

The US has fairly solid insights into the Russian military and foreign ministry, the sources familiar with the intelligence told CNN. Many of those personnel have been involved in the current buildup near Ukraine, which has afforded the US better-than-usual visibility into Russian planning. But the US intelligence community is still largely cut off from Putin and the most senior Russian officials, who are themselves typically isolated from the lower-level operatives carrying out Kremlin orders.

Even so, many of the preparations have been overt and easier to spot. Russia has continued to build up its forces near Ukraine’s borders, including more advanced deployments in Belarus and Crimea, according to satellite images taken by Maxar Technologies and provided to CNN. New housing and personnel camps have sprung up in the last two weeks, the images show. And the US has seen indications that Russia had begun erecting supply lines such as medical units and fuel that could sustain a drawn-out conflict should Moscow choose to invade, officials have previously told CNN.

“Given the type of forces that are arrayed … if that was unleashed on Ukraine, it would be significant, very significant, and it would result in a significant amount of casualties,” Chairman of the Joint Chiefs of Staff Mark Milley told reporters last week. “You can imagine what that might look like in dense urban areas, along roads, and so on and so forth. It would be horrific. It would be terrible. And it’s not necessary. And we think a diplomatic outcome is the way to go here.”

Another US official assessed that the diplomatic path is narrower than ever, however. “We are in Hail Mary territory,” he said.

CNN’s Alex Marquart and Barbara Starr contributed to this report.