CNN has learned the main "stumbling blocks" between the US and Ukraine regarding the framework for ending the war: what it's about CNN — While US officials are increasingly focused on how the war in Ukraine might end, discussions with Ukrainian officials reveal some key differences between Washington and Kyiv on what a possible framework for ending the war might look like, multiple sources familiar with the discussions told CNN. These differences are not causing a rift between the two governments, sources say, and officials acknowledge that Ukraine ultimately makes its own sovereign decisions. But they highlight the difficult calculations facing the Biden administration as it attempts to manage a war with no clear end in sight. One of the main stumbling blocks, sources say, is the issue of what might constitute a victory for Ukraine – and, conversely, what might be seen as an unacceptable outcome that prolongs the conflict. US officials have made it clear to the Ukrainians, according to multiple sources familiar with the discussions, that they do not believe Ukraine is capable of fully regaining all its territory, including Crimea. The US side believes that attempting to do so would cross a red line for Putin, which could lead to a broader war. But Ukrainian officials have told the US that they have no intention of stopping fighting until they reclaim all their territory, including Crimea, sources said. Ukrainian officials see it as an existential issue and argue that any outcome short of that would leave Ukraine vulnerable to future Russian aggression. This fundamental disagreement over war aims has complicated discussions about potential diplomatic solutions, sources said. While the US is not pushing Ukraine to negotiate with Russia at this time, US officials believe it is important to begin exploring potential frameworks for a future settlement. Another stumbling block is the issue of security guarantees for Ukraine after the war. Ukrainian officials have sought strong security guarantees from the US and its allies, similar to those enjoyed by NATO members. But the Biden administration has been hesitant to offer such guarantees, fearing that they could draw the US into a direct conflict with Russia. Instead, the US has discussed providing Ukraine with ongoing military assistance and other forms of support to deter future aggression. These differences over war aims and security guarantees have led to some tense moments in the discussions between the US and Ukraine, sources said. But both sides remain committed to working together to find a way to end the war. "We are in constant dialogue with the Ukrainians about all aspects of the war," a senior administration official told CNN. "That includes their military needs, their economic situation, and their diplomatic goals. We believe it is important to be clear with them about our assessments and our limitations, but ultimately it is up to them to decide how to proceed." Ukrainian officials declined to comment on specific details of the discussions, but emphasized that they appreciate the US support and are committed to working together to achieve a just and lasting peace. "We are grateful to the United States for their unwavering support," said a Ukrainian official. "We are working closely with our American partners to find a way to end the war on terms that are acceptable to Ukraine and that ensure our long-term security."