By Patricia Zengerle, Idrees Ali and Doina Chiacu
WASHINGTON (Reuters) -Russia's war in Ukraine is at a "bit of a stalemate" and Russian President Vladimir Putin appears to be preparing for a long conflict, top U.S. intelligence officials said on Tuesday.
Russia, which calls the invasion "a special military operation," poured more troops into Ukraine for a huge offensive last month in the eastern part of the country but its gains have been slow.
Russia's assault on Kyiv was beaten back in March by Ukrainian resistance.
"The Russians aren't winning and the Ukrainians aren't winning and we're at a bit of a stalemate here," Lieutenant General Scott Berrier, the head of the Defense Intelligence Agency, said before the Senate Armed Services Committee.
He added that so far, between eight and 10 Russian generals have been killed.
Pentagon spokesman John Kirby said that while Russia had made no major gains in Donbas, its troops were making incremental progress.
The war has claimed thousands of civilian lives, sent millions of Ukrainians fleeing and reduced cities to rubble. Moscow has little to show for it beyond a strip of territory in the south and marginal gains in the east.
Putin exhorted Russians to battle in a defiant Victory Day speech on Monday but was silent about plans for any escalation in Ukraine despite Western warnings that he might use his Red Square address to order a mobilization.
During the same hearing, Director of National Intelligence Avril Haines said a Russian victory in the Donbas region in eastern Ukraine might not end the war.
"We assess President Putin is preparing for a prolonged conflict in Ukraine during which he still intends to achieve goals beyond the Donbas," Haines told lawmakers.
She said the United States had indications that Russia wanted to extend a landbridge to Moldova's breakaway region of Transdniestria.
"Combined with the reality that Putin faces a mismatch between his ambitions and Russia's current conventional military capabilities ... the next few months could see us moving along a more unpredictable and potentially escalatory trajectory," Haines said.
TACTICAL NUCLEAR WEAPONS
Western concern at the risk of nuclear war increased after Putin launched the invasion on Feb. 24 with a speech pointedly referring to Moscow's nuclear forces and warning that any attempt to get in Russia's way "will lead you to such consequences that you have never encountered in your history."
Russia said last month it plans to deploy its newly tested Sarmat intercontinental ballistic missiles, capable of mounting nuclear strikes against the United States, by autumn.
Asked about the prospect of Putin using tactical nuclear weapons, Berrier said: "Right now, we do not see that."
Haines said earlier that the intelligence community believes Putin would authorize the use of nuclear weapons only if he perceived an existential threat to the Russian state.
U.S. President Joe Biden on Monday said he was worried Putin did not have a way out of the Ukraine war. Biden said he was trying to figure out what to do about that.
(Reporting by Patricia Zengerle, Idrees Ali and Doina Chiacu; Editing by Mark Porter and Howard Goller)