Several US officials believe President Vladimir Putin is so angry at the resistance Russian troops have been met with in Ukraine that he may launch a more aggressive attack on the neighboring country.

Since invading Ukraine last week following months of threats, Russian troops have attempted to seize the capital city of Kyiv, but have been met with staggering resistance from Ukrainian forces and civilians alike.

Putin's aggression toward Ukraine has been widely condemned by the international community, including with economic sanctions and NATO troops massing in the region.

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As a result of the unforeseen resistance, Putin has reportedly been left confused, frustrated, and directing bursts of anger at people in his inner circle, NBC News reported, citing intelligence from US officials.

"The main concern is the information he's getting and how isolated he is. The isolation is a really big concern," a Western diplomat told NBC News. "We don't believe he has a realistic understanding of what's going on."

US officials also told reporters on Monday that they fear Putin will escalate his invasion following the lack of success in the first six days of the invasion.

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"They have been slowed and they have been frustrated by their lack of progress on Kyiv, and one of the things that could result is a reevaluation of their tactics and the potential for them to be more aggressive and more overt in both the size and the scale of their targeting of Kyiv," a senior defense official told reporters on Monday, per CNN.

Meanwhile, another Western official told Insider on Monday: "There are some risks that come as a consequence of Russia's lack of progress.

"I fear the way in which Russia is frustrated in achieving its aims on the ground is leading to the use of more indiscriminate fires and as a consequence, we're going to see an increase in civilian casualties."

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US officials also warned the country's lawmakers in a classified briefing on Monday that Russia could put Kyiv under siege and that the situation would deteriorate quickly, CNN reported.

"We're certainly mindful that the frustrations could lead to a more aggressive approach by the Russians," a US official told reporters Monday, per The Daily Beast.

"One of the things that could result is a reevaluation of their tactics and the potential for them to be more aggressive and more overt in both the size and scale of their targeting of Kyiv."

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'Angry' Vladimir Putin may order escalation of violence in Ukraine, US officials warn