A senior US defense official says Russia's advance on the Ukrainian capital, Kyiv, has been slower than the Kremlin anticipated, due to stiff resistance.

Citing a Pentagon analysis, the official says Russia's three-pronged invasion continues, including along a route from neighboring Belarus to Kyiv. It's believed Moscow aims to gain control of heavily populated areas.

Speaking on condition of anonymity, the official told reporters on Friday that none of the population centers have fallen, and Ukraine's air defenses are still operating. But the situation could change rapidly.

Meanwhile, the person who was in charge of Ukraine policy under the former Trump administration, Kurt Volker, says Ukraine's military is stronger than in 2014, when Russia annexed the Crimea region.

Volker said Ukrainians now have better equipment and capabilities than they had at the time. One of the armaments that the US has provided to Ukraine is the Javelin anti-tank missile. Volker helped arrange the missile deal.

The Javelin is a precision weapon that is strong enough to penetrate tank armor but portable enough that it can be carried by a single soldier.

Volker said Ukrainian soldiers have encountered sporadic fighting with Russian troops in the country's eastern region. But he acknowledged Russia's overwhelming military superiority, as well as Ukraine's lack of air defenses, coast guard forces, and cyber-attack response capabilities.