Two Russian Ilyushin Il-76 military transport planes were shot down in separate incidents, according to US officials, and there were reports of invading tanks and armored vehicles running out of fuel.
An intelligence update from the Ministry of Defense said Russian forces had continued their advance on Kyiv, with the bulk of their forces now about 18 miles from the center of the city.
However, the MoD noted Russia had yet to gain control of the airspace over Ukraine, greatly reducing the effectiveness of the Russian Air Force.
“Ukrainian Armed Forces continue to put up staunch resistance across the country,” the MoD said.
“Russian casualties are likely to be heavy and greater than anticipated or acknowledged by the Kremlin.”
A statement from the Ukraine military said the first Russian transport plane was shot down on Friday.
Writing for The Telegraph, Mr Heappey states: “After three days of intense fighting, spurred by dogged Ukrainian resistance, Russia is well behind its planned timeline.
“Progress to Kyiv has been much slower than they’d expected, they were unable to take key cities early and now must try to bypass them.
“This leaves pockets of well-armed and well-trained Ukrainians to the rear of the Russian front line, exposing a vulnerable logistics tail – an omen for what awaits Putin.”
Russian soldiers tried to bypass the northern city of Chernihiv, where they were “rebuffed” by the Ukrainian army, in order to attack Kyiv, according to the Ukrainian army.
On Saturday, Mikhail Matveyev, a member of the State Duma, called on the Kremlin to stop the invasion. Andrey Kortunov, an adviser to the Russian foreign ministry, said many officials were “devastated to see what is happening”.
Mr Heappey adds: “The effectiveness of the Ukrainian resistance comes down to one thing – the grit and determination of the Ukrainian people to stand their ground. They still face much more violence and destruction with almost the complete might of the Russian armed forces bearing down on them, but their early success has emboldened the population at large.
“Recruitment centers are now drawing long queues of volunteers wanting to stand up for their country’s freedom. President Putin will have seen this too – if he manages to win the first combat phase, a protracted, bloody, and committed resistance campaign awaits.”