Residents of Ukraine’s capital have been bracing for a massive Russian convoy of tanks and armored vehicles to unleash on the city. That convoy, however, has been stalled for three days just miles outside of Kyiv. 

"They haven't … from our best estimates have not made any appreciable progress geographically speaking in the last 24 to 36 hours, and again nothing very significant. It is difficult for us to know with great specificity all that is going into this stall if you will," Pentagon Press Secretary John Kirby said Wednesday. 

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The convoy, which is about 40 miles long, is stalled about 20 miles outside of Kyiv. The menacing convoy could encircle the capital and prevent anyone from leaving, laying siege on the key city. 

Kirby said that the convoy’s holdup could come down to three reasons: Russians are "deliberately" reassessing their progress and strategizing how to make up for lost time; unanticipated logistical challenges are besetting the Russians’ plan.

"And three, they are getting resistance from the Ukrainians. We have some indications - nothing that we can 100 percent independently verify - but we have some indications that the Ukrainians have in fact tried to slow down that convoy. And we have no reason to doubt those reports, but again we can't speak to it in great specificity," Kirby said. 

Ukrainian forces have allegedly struck the convoy using Ukraine Su-24 and Su-25 fighter jets, as well as with missile barrages, Ukrainian Brig. Gen. Kyrylo Budanov told Military Times on Wednesday morning

"We are striking the enemy’s columns," Budanov said. "We burn many columns of the enemy."

"My intelligence officers and agents are directing and calling the strikes," he said.

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The United Kingdom Ministry of Defence determined Thursday that the convoy has stalled in part by mechanical breakdowns. 

"The main body of the large Russian column advancing on Kyiv remains over 30km from the center of the city having been delayed by staunch Ukrainian resistance, mechanical breakdown and congestion," the ministry said in a tweet.

Reports have also surfaced that the Russian troops are facing food and fuel shortages, which may be contributing factors as to why the convoy has not moved closer to Kyiv. 

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Ukrainian President Volodymyr Zelenskyy said in a Facebook post Wednesday night that morale among Russian fighters has plummeted, and he claimed some are fleeing Ukraine. 

"More and more occupiers are fleeing back to Russia, from us, from you ... we are a nation that broke the enemy's plans in a week — plans those have been built for years," he said.

Ukrainian President Volodymyr Zelensky

BRUSSELS, BELGIUM - MARCH 1: Ukrainian President Volodymyr Zelensky is seen on a screen as he delivers a speech in front of the Assembly of the European Parliament on March 1, 2022 in Brussels, Belgium. Ukrainian President Volodymyr Zelenskyy declared: 'We pay such a price to have the same rights as you'. (Photo by Thierry Monasse/Getty Images)

Despite the convoy not moving, Kyiv and Kharkiv have come under constant attacks from Russian forces.

Kyiv and Kharkiv are "continually under assault," but with no "appreciable movement by the Russians to take" control of those areas, a U.S. defense official said Wednesday.

Explosions rocked Ukraine’s capital early Thursday morning. Zelenskyy’s office attributed one explosion to a missile strike near the capital city’s southern railway station. 

Russian troops have continued battering Ukraine’s second-largest city, Kharkiv, including with aerial attacks. Kharkiv Mayor Igor Terekhov said the city is still standing strong. 

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Russian forces have also laid siege on two Ukrainian seaports, Kherson and Mariupol, while residents of seaport Odessa are bracing for a major attack. 

As the fighting continues, the number of people fleeing Ukraine topped 1 million Thursday, according to the United Nations’ refugee agency. The number marks the swiftest refugee exodus this century.

The Associated Press and Fox News’ Audrey Conklin contributed to this report.