As Ukraine prepares for its own offensive in the coming months, Russian President Vladimir Putin still likely aims to prolong the conflict until Western support for Kyiv wanes, said Director of National Intelligence Avril Haines.

"Putin probably has scaled back his immediate ambitions to consolidating control of the occupied territory in eastern and southern Ukraine, and ensuring that Ukraine will never become a NATO ally," Haines told a hearing of the Senate Armed Services Committee.

But she said that whatever the outcome of Ukraine's offensive -- whether it recaptures much of the occupied south and east or the current stalemate remains -- Putin is unlikely to offer any concessions to advance peace talks.

"The challenge is that even as Putin may be scaling back his near-term ambitions, the prospect for Russian concessions to advance negotiations this year will be low unless domestic political vulnerabilities alter his thinking," Haines said.

While Western allies bolster Kyiv with arms, ammunition and training in advance of its planned offensive, Haines noted that Russian forces have "significant" shortfalls in ammunition and personnel.

"Even if Ukraine's counter-offensive is not fully successful, the Russians are unlikely to be able to mount a significant offensive operation this year," Haines said.

To sustain even a modest offensive campaign, Putin would have to order a mandatory mobilization of army recruits as well as secure significant ammunition supplies from other countries, beyond what it gets from Iran, she said.

Still, she added: "We continue to assess that Putin most likely calculates that time works in his favor and that prolonging the war may be his best remaining pathway to eventually securing Russia's strategic interests in Ukraine."

Haines said Moscow's losses are such that its military requires "years of rebuilding."

With a weakened conventional military force, Moscow will become more reliant on nuclear, cyber and space capabilities for deterrence, she said.

Haines added it was "very unlikely" that Moscow would use nuclear weapons in the conflict.