Russian President Vladimir Putin still wants to capture most of Ukraine, US intelligence agencies believe.
Moscow's troops have been so weakened by combat, however, that US officials assess they are only capable of making slow territorial gains, reports BBC.
It means the war could last for a long time, Director of National Intelligence Avril Haines says.
In March Moscow refocused its efforts on seizing Ukraine's Donbas area after failing to take Kyiv and other cities.
Mr Putin still has the same goals as the ones he held at the start of the conflict, the US's top intelligence officer Ms Haines said - to take most of Ukraine.
But, she says, Russia is unlikely to achieve that goal any time soon.
"We perceive a disconnect between Putin's near-term military objectives in this area and his military's capacity, a kind of mismatch between his ambitions and what the military is able to accomplish," she told a US Commerce Department conference.
Long-running war
In her first public comments of US intelligence on the war since May, Ms Haines' suggested Russia's invasion will grind on "for an extended period of time" and that "the picture remains pretty grim".
The most likely scenario would see Russian forces remaining preoccupied with the conflict for a long time, she said, limiting Moscow's wider military capabilities.
"Russian troops are unlikely to be able to conduct multiple simultaneous operations" for as long as the war grinds on, Ms Haines said, based on US intelligence assessments.
It may mean Moscow becomes more dependent on "asymmetric tools" to target its enemies; including cyber-attacks, efforts to control energy resources and even nuclear weapons.
Ms Haines' comments came on Wednesday after Nato leaders pledged to stand behind Ukraine for as long as it takes - boosting their troop presence across Europe and inviting Finland and Sweden to join the group.
Responding to the possibility of the two Nordic countries becoming Nato members, Mr Putin accused the military alliance of deliberately escalating tension.
"If Nato troops and infrastructure are deployed, [Russia] will be compelled to respond," Mr Putin said while on a trip to Turkmenistan.