Russian forces have moved about three miles closer to Kyiv in the past 24 hours, according to a senior US defence official.
After days of being stalled, Russia's military could be as close as nine miles (15km) from the city centre.
According to the defence official, here is what has changed in the past 24 hours:
• Russian forces have advanced on the northwest route into Kyiv – near Hostomel Airport (also known as Antonov Airport)
• East of the capital, they are also advancing along two parallel lines
• Some troops may have turned back to Sumy, but it is not clear why
• Chernihiv is now isolated, but the Russian military has not moved past this city – with heavy fighting still reported, and Ukrainians fighting back
• Mariupol is encircled but not yet in the hands of the Russians, who are inching closer
The unidentified official also said that, while Ukraine still has most of its fixed wing fighter aircraft available, these are not being used very much.
Instead, Ukrainians are focusing on their own surface to air systems, man-portable air-defence systems (MANPADs), and anti-armour systems.
Russia, meanwhile, is relying on long-range fires, bombardment from aircraft and mobile launchers on the ground, with the US saying that this is due to their lack of achievement on the ground.
Earlier on Thursday, the UK Ministry of Defence said: "Due to strong resistance, Russian forces are committing an increased number of their deployed forces to encircle key cities.
"This will reduce the number of forces available to continue their advance and will further slow Russian progress."
In other developments, the satellite image company Maxar says new images show that a large Russian convoy that was headed for Kyiv and near Antonov Airport has largely dispersed and redeployed.
Photographs suggest armoured units are now manoeuvring in and through surrounding towns close to this airport.
Separately, there have been reports of internet outages around the city of Kherson – and the US estimates that Russia has fired more than 775 missiles since the beginning of the invasion.