Lviv, Ukraine and Washington, DC(CNN) Russia's invasion of Ukraine is encountering "stiffer than expected" resistance from the Ukrainian military as well as unexpected difficulties supplying its forces, two senior US officials with direct knowledge tell CNN.
On the battlefield, Russia is suffering heavier losses in personnel and armor and aircraft than expected. This is due in part to the fact that Ukrainian air defenses have performed better than pre-invasion US intelligence assessments had anticipated. In addition, Russia has yet to establish air supremacy over Ukraine, a senior dedfense official said, as the Ukrainian Air Force and air defense systems fight for control of the airspace.
"Ukrainian air defenses, including aircraft, do continue to be operable and continue to engage and deny access to Russian aircraft in places over the country," the official said.
Without uncontested control of the skies, it becomes more difficult for an army on the move to see and strike targets from the air.
Together, these challenges have so far prevented the quick overthrow of major Ukrainian cities, including the capital, Kyiv, which US officials were concerned could play out in a matter of days. The city of Kharkiv near Ukraine's border with Russia also has not fallen to invading forces, which officials worried could happen on the first night of an invasion.
Officials caution that this picture of the battlefield is just a moment in time, and the situation on the ground could change very quickly as Russian forces keep up their assault.
These officials noted that Russian forces still greatly outnumber Ukrainian forces, and Russia continues to maneuver these forces into position around major urban centers. It's also unclear how much of the slower movement can be attributed to the logistical challenge of moving such a large force.
The Ukrainian military has a number of different anti-aircraft weapons, including radar-guided and heat-seeking missiles, as well as anti-aircraft guns, according to IHS Janes. The US has also provided the Ukrainian military with Stinger anti-aircraft missiles in recent weeks, as have other NATO allies.
As of Saturday evening in Ukraine, the US had not seen any indication that the Russian military has taken control over any Ukrainian cities, the official said, even as Russian forces have moved to surround some population centers, including Kyiv.
Keeping Russia's large invasion force supplied with fuel and ammunition has also proved difficult. As one senior US official explained, Russia anticipated a fast victory and may have neglected to plan for sufficient resupply of its forces. Supply lines, this official explained, are a "definite vulnerability."
"What we assess now is that he had to commit a bit more logistics and sustainment, in fuel specifically, than what we believe they had planned to do this early in the operation," said one of the officials.
Russia's forces launched their invasion of Ukraine early Thursday morning, with missiles landing in cities across the country. US officials have said Moscow's goal is to take Kyiv and "decapitate" the government led by Ukrainian President Volodymyr Zelensky.
Zelensky turned down an offer from the US to evacuate from Kyiv, the Ukraine Embassy in Britain wrote Saturday on Twitter. Zelensky has posted videos on social media showing he's remained in Ukraine.
"The world has seen that Ukrainians are powerful, Ukrainians are courageous, they're on their native land, and they're never going to give it up to anyone. They will never betray it," he said in an address Saturday.
Russia has spun their slow advance in Ukraine as a stop to allow time for negotiations, not a military setback.
On Saturday, the Russian Ministry of Defense said its troops have been ordered to resume their offensive "in all directions," after a suspension was ordered for negotiations with the Ukrainian government. The ministry said the offensive was ordered to continue after Ukraine abandoned the consultations.
A Ukrainian presidential adviser denied in the early hours Saturday that Ukraine had refused to negotiate.
"They are having problems," a NATO official said of the Russian forces, pointing to the alliance's latest intelligence. "They lack diesel, they are proceeding way too slow and morale is obviously an issue."
Asked whether the Russians are likely to intensify their efforts, the official said they have no choice. "They are way behind schedule," the official said. "This is getting out of hand for them, every additional day is very painful."
Former Ukrainian President Petro Poroshenko, who has also remained in Kyiv, said in an interview Saturday on CNN that the Ukrainian military has displayed "exceptional heroism" but that the country still needs more help from its allies in receiving weapons like anti-tank missiles.