The U.S. intelligence chief says Putin is preparing for a prolonged conflict.

The U.S. director of national intelligence said on Tuesday that Russia’s goals in the war remained expansive, including the creation of a land bridge across Ukraine’s Black Sea coast, but that the Kremlin would struggle to achieve those gains without building up its military with a large-scale mobilization or draft.

The official, Avril D. Haines, told the Senate Armed Services Committee that the next month or two of fighting will be significant as President Vladimir V. Putin of Russia tries to reinvigorate his military campaign. But she said that even if Russia were successful in taking the Donbas region of eastern Ukraine, it would 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,” Ms. Haines said.

Ms. Haines said American intelligence agencies did not believe that Moscow could exert control over Donbas and the buffer zone Russia wants to establish — across Ukraine’s Black Sea Coast all the way to the Transnistria region of Moldova — in the coming weeks. But she said the Russian leader was pursuing a longer-term aim.

“Putin most likely also judges that Russia has a greater ability and willingness to endure challenges than his adversaries,” Ms. Haines said. “And he is probably counting on U.S. and E.U. resolve to weaken as food shortages, inflation and energy prices get worse.”

Lt. Gen. Scott D. Berrier, the director of the Defense Intelligence Agency, told the Senate committee that neither side was winning.

“The Russians aren’t winning, and the Ukrainians aren’t winning, and we’re at a bit of a stalemate here,” General Berrier said.

That stalemate, he said, could last for a while. Under questioning from Senator Tom Cotton, Republican of Arkansas, General Berrier said Ukraine was better able to generate additional combat power. But if Russia declares a full-scale war, and widespread conscription of its population, it could bring thousands more soldiers into the fight, General Berrier said.

Ms. Haines said that with both Ukraine and Russia believing they can make military progress, a diplomatic solution did not seem viable in the short term.

“The reality that Putin faces a mismatch between his ambitions and Russia’s current conventional military capabilities likely means the next few months could see us moving along a more unpredictable and potentially escalatory trajectory,” Ms. Haines said.

Current trends, she said, increase the possibility that Mr. Putin will impose martial law, reorient industrial production or take escalatory military actions, which she suggested could include more nuclear threats.

If he believes such threats are being ignored, Ms. Haines said, he could authorize a major nuclear exercise, although the United States continues to believe that there is not an imminent threat that Mr. Putin would authorize the use of a nuclear weapon.

Washington, she said, “will remain vigilant and monitoring every aspect of Russia’s strategic nuclear forces.”