Mr. Putin sees Crimea as a territory of great strategic and historical importance. At the same time, Ukraine, the United States and European nations have insisted for years that Crimea’s status is nonnegotiable. Biden administration officials have repeatedly said that one of the main reasons for supporting Ukraine is to defend the core principle that borders cannot be changed by force.
“In terms of the ultimate status of Crimea, that will be something to be negotiated or discussed between the Ukrainians and the Russians, but Crimea is Ukraine,” Colin H. Kahl, the Pentagon’s under secretary for policy, said on Tuesday.
Ukrainian military advances on Crimea, though a distant prospect for now, would stoke concern in Washington about Mr. Putin’s threats to escalate the conflict.
American and European leaders see their goal for now as keeping a protracted war contained to Ukraine and deterring Mr. Putin from using a tactical nuclear warhead or other weapon of mass destruction. Officials debate whether Mr. Putin is bluffing when he hints at using nuclear arms, but some analysts believe that control of Crimea, home to Russia’s Black Sea Fleet, could be a red line for the Russian leader.
American officials have said for months that they are sending private and public messages to the Kremlin to warn of severe consequences if Mr. Putin uses nuclear weapons. Mr. Sullivan has been talking to Nikolai Patrushev, his Russian counterpart, since the beginning of the war to try to avert any misunderstandings around nuclear threats, the Biden administration official said.
“I have known both Jake Sullivan and Tony Blinken for years,” said Representative Ro Khanna, Democrat of California, referring to Secretary of State Antony J. Blinken. Mr. Khanna, who was among those who signed the progressives’ letter to Mr. Biden, continued, “I have confidence that they understand the risks of nuclear war and the risks of escalation, and are doing everything they can to stand with Ukraine while minimizing the risks of the conflict escalating.”
American officials said Mr. Zelensky’s private position has been the same as his public one: He wants to see Ukraine’s pre-2014 territory returned, and he is not interested in trading any of it for an end to the war.