Ukraine's challenges in massing troops, ammunition and equipment could cause its military to fall "well short" of Kyiv's original goals for an anticipated counteroffensive aimed at retaking Russian-occupied areas this spring, according to U.S. intelligence assessments contained in a growing leak of classified documents revealing Washington's misgivings about the state of the war.

Labeled "top secret," the bleak assessment from early February warns of significant "force generation and sustainment shortfalls," and the likelihood that such an operation will result in only "modest territorial gains." It's a marked departure from the Biden administration's public statements about the vitality of Ukraine's military and is likely to embolden critics who feel the United States and NATO should do more to push for a negotiated settlement to the conflict.

The document, which has not been previously disclosed, is among a trove of U.S. national security materials discovered last week on an online messaging platform. Both the Pentagon, where much of the leaked materials appear to have originated earlier this year, and the Justice Department are investigating the matter.

The leak has produced remarkable insights into U.S. intelligence activities worldwide, but its revelations about the Russia-Ukraine war have proved particularly illuminating. It has revealed, for instance, where American officials have detected critical weaknesses in Ukraine's air defenses and access to ammunition while exposing considerable deficiencies within the Russian military, too. Many of the assessments date to February and March.

The document forecasting only modest success in Ukraine's forthcoming counteroffensive indicates that Kyiv's strategy revolves around reclaiming contested areas in the east while pushing south in a bid to sever Russia's land bridge to Crimea, the peninsula Moscow illegally annexed in 2014 and now uses as a supply route for its forces inside Ukraine. The potency of entrenched Russian defenses coupled with "enduring Ukrainian deficiencies in training and munitions supplies probably will strain progress and exacerbate casualties during the offensive," the document says.

Its markings suggest the information was supplied by human and signals intelligence, likely involving sensitive methods used by the CIA and the National Security Agency. The Office of the Director of National Intelligence, which appears to have produced the leaked document, declined to comment, as did the National Security Council. The Defense Department declined to address the document's contents.

At the Pentagon on Monday, spokesman Chris Meagher said officials were moving aggressively to determine the leak's scope, scale and impact, and how they can prevent similar incidents in the future. He declined to comment on the materials' veracity. A State Department spokesman, Vedant Patel, said U.S. officials also are working to reassure allies and partners "of our commitment to safeguarding intelligence."

Beyond the leaked document, U.S. officials said the prospects for a modest outcome in the spring offensive also were reinforced in a classified assessment by the National Intelligence Council. That assessment, which was recently briefed to a select group of people on Capitol Hill, found that Ukraine was unlikely to recapture as much territory as Kyiv did last fall in Ukraine's stunning breakthroughs in the east and south, people familiar with the matter said.

In the weeks since the leaked document was drafted, U.S. officials have held talks with Ukrainian leaders to ensure that Kyiv's ambitions for the offensive match its capabilities, said U.S. officials who, like others interviewed for this report, spoke on the condition of anonymity to discuss the sensitive matter. One senior-level exchange occurred in mid-March during a call among Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff; Jake Sullivan, the president's national security adviser; Defense Secretary Lloyd Austin; and their Ukrainian counterparts.

U.S. officials also have held tabletop exercises with Ukrainian military leaders to demonstrate how different offensive scenarios could play out, and the consequences of spreading forces too thin, one official said. This could stretch supply lines too far, making it difficult to hold retaken territory while trying to push further into occupied areas.

All parties came away from those conversations with a sense that Ukraine was beginning to understand the limitations of what it could achieve in the offensive and preparing accordingly, U.S. officials said. While severing the land bridge is unlikely to happen, these people said, the United States is hopeful that incremental gains could at least threaten the free flow of Russian equipment and personnel in the corridor, which has been a lifeline for invading forces.

A senior Ukrainian official did not dispute the revelations in the document and pointed to logistical backlogs that have slowed promised deliveries of Western aid. It is "partially true," the official said, "but the most critical part is a delay of the already promised systems, which delays training of newly formed brigades and the counteroffensive as a whole." Several nations, including the United States, have committed battle tanks and other armored vehicles to Ukraine but only after agonizing over the decision to do so, drawing criticism from Kyiv and its staunchest backers in Eastern Europe. Washington has accelerated plans to send a bloc of Abrams tanks, but delivery remains months away.

Another senior Ukrainian official said the leaked documents were unlikely to compromise the planned counteroffensive. "Everyone knows we're low on ammunition—the president and the defense minister talk about that openly," the official said. "And it's been obvious to everyone since November that the next counteroffensive will be focused on the south, first Melitopol and then Berdyansk. But the exact place—we can change that the week before."

Military officials and independent analysts have publicly suggested the likelihood of a counteroffensive through Ukraine's east and south. Russia has bolstered its defense of the Crimean Peninsula with a dense web of fortifications and trench lines in apparent anticipation of such an operation.

Moscow's troops also face considerable challenges, including low morale in the wake of major strategic blunders that have lead to significant casualties and left pockets of soldiers poorly equipped. Russian units in the east are particularly shabby, according to a separate document contained in the U.S. leak, with intelligence revealing substandard care in camouflaging sensitive sites and reckless ammunition storage.

The West has sent Ukraine tens of billions of dollars' worth of weapons and military equipment, touting the injection of aid as a significant boost. But the newly leaked document signals what many commanders and troops already know: The difficult fight against Russia has exhausted Ukraine's troops and hardware, making every day the war drags on an advantage to the larger Russian military.

Ukrainian units are burning through historic levels of artillery ammunition and have begun rationing shells, according to soldiers. Artillerymen supporting operations in the embattled city of Bakhmut, for instance, have said their aging Soviet howitzers are less accurate than Western guns, requiring them to fire more and wear down their equipment faster. Soldiers are just as worn out, and Kyiv has reached deeper into its population to mobilize additional fighters, sometimes stopping men on the street to hand them draft papers.

Military personnel in the field have in recent weeks complained that newly mobilized troops arriving on the front lines are poorly trained. But the situation on the battlefield now may not reflect a complete picture of Ukraine's forces, because Kyiv is training troops for the coming counteroffensive separately—deliberately holding them back from the current fighting, including the defense of Bakhmut, a U.S. official said.

The prospect of pouring billions of dollars into a military stalemate with only incremental gains in one direction or another could weaken the resolve of Kyiv's backers in Europe and in the United States, possibly sharpening calls for negotiations between Kyiv and Moscow.

But opening talks with Russian President Vladimir Putin could be risky for Ukraine's leader, President Volodymyr Zelensky, given the acute animosity toward the Kremlin among the Ukrainian people, who have suffered extraordinary levels of violence and hardship during the conflict yet have held together with the promise of achieving a total victory.

Khurshudyan reported from Kyiv, Ukraine. The Washington Post's Serhiy Morgunov in Kyiv, and Dan Lamothe and Ellen Nakashima in Washington contributed to this report.