The grinding war between Ukraine and Russia is expected to bleed into 2024 with neither side securing victory yet both refusing to negotiate an end to the conflict, according to a Defense Intelligence Agency assessment that is among the highly sensitive U.S. government materials leaked online and obtained by The Washington Post.

The analysis concludes that, even if Ukraine recaptures "significant" amounts of territory and inflicts "unsustainable losses on Russian forces," an outcome U.S. intelligence finds unlikely, the nation's gains would not lead to peace talks.

"Negotiations to end the conflict are unlikely during 2023 in all considered scenarios," says the document, which has not been disclosed previously.

The assessment, based on close U.S. scrutiny of each side's troop counts, weaponry and equipment, could galvanize the war's critics who have called on major powers such as the United States and China to push for Kyiv and Moscow to reach a settlement and end a conflict that has displaced millions and left hundreds of thousands dead or wounded.

Asked about the DIA's assessment, a U.S. official said the decision on when to negotiate is up to President Volodymyr Zelensky and the Ukrainian people, underscoring what has been a hands-off approach to mediation espoused by the administration since Russia's full-scale invasion began in February 2022. The United States will continue to stand with Kyiv and provide it with the equipment and weapons that will bolster its position at the negotiating table, whenever that day comes, the official said.

The document leak, which first came to authorities' attention last week, has provided extensive insight into U.S. intelligence activities worldwide and exposed the national security establishment's deep misgivings about the war's trajectory. Many of the classified assessments that have surfaced thus far date to February and March, first appearing on the Discord messaging platform before spreading elsewhere online. Both the Pentagon, where many of the leaked materials appear to have originated earlier this year, and the Justice Department have said they are investigating the matter.

The Office of the Director of National Intelligence declined to comment. Spokespersons for the Russian and Ukrainian governments did not respond to requests for comment.

Beyond forecasting a costly open-ended conflict, the newly disclosed document also predicts how Ukrainian and Russian military leaders will respond to battlefield challenges, and it anticipates that the year will end with the two sides achieving only "marginal" territorial gains as a result of "insufficient troops and supplies for effective operations."

Such a stalemate, where neither side achieves a decisive advantage, is described in the document as "the most likely scenario."

For the Ukrainian side, an ongoing war of attrition will lead to frustration within the country and "criticism" about how the war is conducted, "making leadership changes more likely," the document says.

It is unclear if the document is referring to leadership changes in a political or military context. Zelensky remains broadly popular in Ukraine, but tensions exist between his office and Gen. Valery Zaluzhny, head of Ukraine's armed forces, whom some in Kyiv view as a political threat.

A stalemate also will result in Ukraine enacting the "full mobilization" of its remaining eligible population, the document predicts, sending more young men to the front lines. At the same time, Ukraine probably will intensify its reliance on strikes in Russian territory, the document says, a dynamic that has disquieted some U.S. officials fearful that such attacks could compel President Vladimir Putin to escalate the conflict or give China cause to begin providing lethal support to Russia.

For the Russian side, the stalemate will force Moscow to employ "degraded reserves due to dwindling combat power," the document says. The Kremlin also is likely to "accelerate" efforts to integrate captured territories into Russia.

"It's always been a race to see who runs out of resources first," said Heather Conley, a Europe scholar and president of the German Marshall Fund.

She said she agreed with the U.S. intelligence contention that negotiations would begin only after one side is "exhausted," a prospect that appears far off.

Jeff Rathke, a scholar at Johns Hopkins University and former U.S. diplomat, said the assessment "reflects a sobriety about the likelihood of either side being able to mount or to mass decisive military power in places where it can achieve strategic effect."

The newly disclosed document also analyzes what might result from Russia or Ukraine achieving a "decisive advantage" on the battlefield. In the event that Russia deals a significant blow against Ukraine and captures more territory, Moscow is likely to "posture forces to achieve further objectives, such as regime change" in Ukraine, it says.

Regime change appeared to be the goal of Russia's invasion, but its forces were thwarted in their blundering attempt to sack the capital.

In the scenario in which Ukraine gains a decisive advantage, however, U.S. intelligence believes that Kyiv is likely to "conduct riskier offensive operations for additional gains." In response, Russia could be expected to "increase nonconventional attacks on Ukraine," though, importantly, "nuclear use remains unlikely," the document says. Officials predict that, rather than giving up, the Kremlin would opt to announce a "new national mobilization" to sustain further combat operations.

U.S. officials have cautioned that such analysis related to the war in Ukraine is fluid, and the materials that leaked may lack nuance the United States has gleaned in the days since they were drafted. The newly disclosed document acknowledges that the stalemate it describes as the most likely scenario by year's send may not hold if there are "substantive improvements to Ukrainian or Russian military capabilities."

Both sides are preparing for fighting to escalate as warmer weather arrives, though officials in Kyiv and personnel on the war's front lines have complained about logistical backlogs responsible for slowing promised deliveries of Western arms. Several nations, including the United States, committed battle tanks and other armored vehicles to Ukraine over the winter while initiating new training pipelines aimed at teaching advanced combat tactics to thousands of Ukrainian soldiers.

All the while, what remains very consistent in the Biden administration's position on Ukraine is its deep skepticism about peace negotiations. Secretary of State Antony Blinken has openly questioned the value of permanent or temporary armistice agreements, telling members of the U.N. Security Council in February that they "should not be fooled by calls for a temporary or unconditional cease-fire." Russia, he said then, will use "any pause in fighting to consolidate control."

Blinken has also criticized countries for urging both sides to negotiate, pointing out that Russia is the aggressor and Ukraine has every right to fight to regain its territory.

Publicly, neither Russia nor Ukraine has ruled out negotiations, but their demands are leagues apart.

The 10-point peace plan Zelensky released last year demands the full withdrawal of Russian troops "from the territory of Ukraine," including Crimea, the peninsula Putin annexed illegally in 2014 that now serves to facilitate the resupply of his forces inside Ukraine.

Moscow, meanwhile, has insisted that Ukraine must acknowledge the new "territorial realities," which has been interpreted to mean that Zelensky and his Western backers must recognize the regions of Zaporizhzhia and Kherson, which Russia does not even fully control, as Russian territory, a non-starter for Ukraine as well as the United States.

China proposed a 12-point peace plan for the conflict with criteria that few found objectionable, but it has gained little traction since it was introduced in February.

The Washington Post's Samuel Oakford contributed to this report.