According to four people familiar with the intelligence, intercepted communications obtained by the US have revealed that some Russian officials are concerned that a large-scale invasion of Ukraine would be more costly and difficult than Russian President Vladimir Putin and other Kremlin leaders realise.

Those officials, according to three sources, include intelligence and military operatives.

Officials have also expressed dissatisfaction with their plans being found and made public by western nations, according to two sources who cited intercepted communications.

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According to two of the sources, there is no proof that these officials are opposed to the overall strategy or would rebel against Putin’s commands. Another US intelligence source said that Russia has a professional military that would be expected to carry out Putin’s commands properly.

Nonetheless, Russian defence officials believe it will be difficult, according to a senior European official. “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 source familiar with the intelligence said the manner Russian strategy has evolved and extended over the last two months indicates that high Russian officials have heard the worries.

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Officials informed CNN on Saturday that Putin has gathered 70% of the military soldiers and weapons needed for a full-scale invasion of Ukraine on the country’s borders. According to some reports, he may be plotting an attack on Kyiv, Ukraine’s capital, and the city may fall within 48 hours of a large-scale land and air invasion.

Another scenario, according to the officials, is that Putin decides on a multi-pronged operation, sending forces in from several directions across Ukraine to swiftly splinter the Ukrainian military’s capacity to fight as a unified force — a traditional Russian military approach.

“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.”

Officials from the United States have stated publicly and privately that they have no idea whether Putin has decided to attack, or what form that attack would take if he did. The reason for this, according to CNN’s sources, is that the US has very little visibility into Putin and his inner circle.

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According to CNN’s sources familiar with the intelligence, the US has reasonably solid insights into the Russian military and foreign ministry. Many of those personnel have been involved in the current buildup near Ukraine, giving the US greater insight into Russian plans than usual. However, the US intelligence agency remains generally shut off from Putin and the highest-ranking Russian officials, who are often segregated from lower-level operations carrying out Kremlin commands.

Despite this, many of the preparations were obvious and easy to notice. According to satellite pictures provided to CNN by Maxar Technologies, Russia has continued to build up its military along Ukraine’s borders, including more advanced deployments in Belarus and Crimea. The photographs suggest that new housing and troop camps have sprouted up in the last two weeks. Officials earlier told CNN that the US had observed signs that Russia has begun establishing supply lines such as medical units and fuel that could sustain a long-term fight if Moscow chose to invade.

“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.”

However, according to another US official, the diplomatic path is narrower than ever. “We are in Hail Mary territory,” he said.