Three U.S. officials have told Newsweek they expect Ukraine's capital Kyiv to fall to incoming Russian forces within days, and the country's resistance to be effectively neutralized soon thereafter.

The officials, speaking on the condition of anonymity, said that Moscow's focus, as revealed in Russian President Vladimir Putin's references to a "special military operation" to "demilitarize" the neighboring country, would be to encircle Ukrainian forces and force them to surrender or be destroyed. They expect Kyiv to be taken within 96 hours, and then the leadership of Ukraine to follow in about a week's time.

And Russia's thunderous attacks on Ukrainian government and military institutions, paired with reports of ground personnel seeking to take strategic points, including the Chernobyl nuclear facility, appeared to only be the initial phase of what may be a more comprehensive ground campaign.

One former senior U.S. intelligence officer with extensive experience dealing with Russia expressed a similar sentiment.

"After the air and artillery end and the ground war really starts, I think Kyiv falls in just a few days," the former senior U.S. intelligence officer told Newsweek on the condition of anonymity as well.

"The military may last slightly longer," the former intelligence officer added, "but this isn't going to last long."

Afterward, the senior U.S. intelligence officer said the next stages may be determined by U.S. President Joe Biden's capability and willingness to risk further provoking Moscow by supporting partisan efforts on behalf of a potential Ukrainian resistance.

"Then it either becomes a robust insurgency or it doesn't, depending largely on Biden," the former senior U.S. intelligence officer said.

A source close to Ukrainian President Volodymyr Zelenskyy's government, who also asked not to be named, agreed with the U.S. assessment that Kyiv could be surrounded within 96 hours. But the source did not believe Zelenskyy's government would collapse.

Asked by Newsweek whether the government was confident it could break a possible Russian encirclement, the source said, "I think it's too early to say...They say Ukraine is holding better than they expected."

A NATO diplomatic official, who also did not wish to be named as the official was not authorized to speak publicly on the subject, told Newsweek of the U.S. assessment: "My personal opinion: unfortunately, it does sound rather believable. However, I think now the first 24 hours are the most critical."

A statement from Mykhailo Podolyak, an adviser to Ukraine's presidential chief of staff, and shared with Newsweek by Ukraine's embassy in Washington outlined what Kyiv suspected were Moscow's goals and the likelihood of Russian forces seizing government buildings in major cities.

"The Office of the President of Ukraine believes the Russian federation has two tactical goals – to seize territories and attack the legitimate political leadership of Ukraine in order to spread chaos and install a marionette government that would sign a peace deal on bilateral relations with Russia," Podolyak said. "The enemy attempts to destabilize [the] situation in large cities, in particular Kharkiv and Kyiv. The probability exists the Russian armed forces will seize the government quarters."

Smoke rises from outside an intelligence building on February 24, 2022 in Kyiv, Ukraine. Overnight, Russia began a large-scale attack on Ukraine, with explosions reported in multiple cities and far outside the restive eastern regions held by Russian-backed rebels. Chris McGrath/Getty Images

The Antonov Airport near the town of Hostomel, just outside Kyiv, was the scene of some of the most dramatic early fighting. Ukrainian Interior Ministry officials reported early Thursday that Russian helicopter-borne forces had seized the airfield, though fighting around it is believed to be ongoing.

The outcome could be pivotal to Ukraine's fate. The airport is 15 miles west of Kyiv. If secured by Russian troops, Antonov could become a springboard for an assault on the capital.

"Let's see if they can counter," the Ukrainian source said of the Ukrainian troops at Hostomel.

Conflicting reports have emerged of whether Russian troops have captured or been repelled from various positions across the country. One notable site was Chernobyl, where the world's worst nuclear disaster took place during the Soviet era in 1986 and had since been captured by Russian forces, according to Podolyak.

But clashes continued as both sides tallied the other's casualties.

The Ukrainian Armed Forces commander-in-chief Valerii Zaluzhnyi claimed earlier Thursday that Ukrainian troops have destroyed at least four Russian tanks and dozens of armored vehicles and downed up to six Russian planes and two helicopters. The Ukrainian Armed Forces then reported that Ukraine was targeted by four ballistic missiles from the direction of Belarus.

The Ukrainian Foreign Ministry's Crisis Center, for its part, reported more than 30 strikes with Kaliber cruise missiles, multiple-launch rocket systems, aircraft and artillery against Ukrainian civil and military infrastructure.

Russian Defense Ministry spokesperson Igor Konashenkov later said that "groups of troops of the Donetsk and Luhansk People's Republics broke through the well-equipped echeloned defence of the Armed Forces of Ukraine, advanced 6-8 kilometers deep," a task "made possible thanks to the fire support of Russian artillery and army aviation."

"All tasks assigned to the groups of troops of the Armed Forces of the Russian Federation for the day have been completed successfully," the spokesperson said.

He added that the "joint use of raiding detachments and airborne troops in the Crimean direction enabled Russian troops to reach the city of Kherson," an effort that "made it possible to unblock the North Crimean Canal and restore water supply to the Crimean peninsula."

"In total, as a result of the strikes of the Russian Armed Forces, 83 ground objects of the military infrastructure of Ukraine were disabled," the spokesperson added. "Since the beginning of the special military operation, two Su-27s, two Su-24s, one helicopter and four Bayraktar TB-2 attack unmanned aerial vehicles of the Armed Forces of Ukraine have been shot down."

The list was later amended to also include Russia-built, Ukraine-operated S-300 and Buk-M1 surface-to-air missile systems, along with 11 airfields three command points and a naval base.

Putin's decision to invade Ukraine came late Wednesday as the Russian leader accused the fellow former Soviet republic of acting as a puppet to Western interests by seeking to join the U.S.-led NATO military alliance and acquire weapons to threaten Russia's national security.

"We have been left no other option to protect Russia and our people, but for the one that we will be forced to use today," Putin said. "The situation requires us to take decisive and immediate action."

Ukrainian officials have repeatedly denied that their country poses any threat to Russia. Amid an unprecedented Russian military buildup along Ukraine's borders, the Biden administration has warned for weeks that an incursion could begin at any time.

Newsweek reported Wednesday, prior to the beginning of the invasion, that the United States had warned Zelenskyy that a full-scale Russian invasion to include the use of airstrikes, missiles, ground troops and cyber attacks was anticipated to begin within 48 hours.

In the weeks and months leading up to the eruption of the conflict, Moscow had accused Washington of hyping up the possibility of a war. On Monday, however, Putin sent in troops he termed "peacekeepers," after offering recognition to two breakaway separatist states in Ukraine's eastern Donbas region, signaling that more military action might follow.

The self-proclaimed Donetsk and Luhansk People's Republics then appealed to the Russian leader for military support, which Putin granted.

The United States and its allies have roundly condemned Putin's actions. They have begun to introduce a series of sanctions designed to hurt Russia's economy and the wealth of influential officials within Putin's inner circle, as well as their family members.

After announcing sanctions against Russia's sovereign wealth fund, Biden added further economic restrictions against state-backed institutions during a televised address Thursday.

"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," Biden said. "Russia alone is responsible for the death and destruction this attack will bring, and the United States and its allies and partners will respond in a united and decisive way. The world will hold Russia accountable.

This is a developing news story. More information will be added as it becomes available.