Radio Free Europe/Radio Liberty/Serhii Nuzhnenko/Reuters
Ukrainian servicemen rest at their positions after a fight, as Russia's attack on Ukraine continues, near the front line city of Bakhmut, in Donetsk region, Ukraine, in May.
CNN  — 

As a Ukraine aid package continues to stall in the US Congress, America and its allies are assessing what they describe as the potentially debilitating impact on Ukraine’s defense and longer-term prospects of losing the war, multiple US and European officials told CNN.

“There is no guarantee of success with us, but they are certain to fail without us,” a senior US military official said.

The most immediate concern is the impact on Ukraine’s ongoing counteroffensive in the east and south, where Ukrainian forces have struggled to make significant forward progress even when US support was still coming. “If looking at taking and holding further territory,” said one European diplomat, “it is hard to see how that could succeed without continued US support.”

More broadly, Western officials fear the loss or further delay of US support will impact aid from its allies. On Friday, Ukraine suffered another blow when Hungary blocked further European Union aid, though talks on the issue are expected to resume in January. The news underlined the scale of the challenge facing Kyiv and many fear that if the US fails to continue providing support, European nations will follow.

“If we go south,” said Democratic Rep. Mike Quigley, who co-chairs the Congressional Ukraine Caucus, “our allies will too.”

Now, Western intelligence agencies are currently calculating how long Ukraine could hold out without US and NATO help. One senior US military official estimated months, with a worst-case scenario of a significant setback or even defeat by the summer. A Russian victory would not just be dire news for Ukraine, it would be disaster for wider European security and a major blow to the US.

Asked about the continued delay in US aid, Estonian Prime Minister Kaja Kallas told CNN from Brussels, “We can’t talk about war fatigue right now because if we do and give in, then Putin wins and that will mean a catastrophe to everybody. That will mean more conflicts, more wars, more scarcity of food supply, all the different worries that come along with it. So that’s why we have to make an effort now.”

Ukrainian forces are already rationing ammunition, US and Ukrainian officials told CNN, as Russian forces fire back at a ratio of five to seven times greater than Ukrainian forces are able to. A senior Ukrainian military official told CNN that Ukrainian commanders believe the impact on their firepower has led to additional Ukrainian casualties.

Without additional US aid, Western officials assess that Ukraine would first run out of long-range missiles, then air defense missiles and later artillery ammunition and short-range missiles such as shoulder-fired Javelin anti-tank missiles and Stinger anti-aircraft missiles.

Each category of munition has been crucial to Ukraine’s defense. The long-range missiles, such as the UK-supplied Storm Shadow cruise missiles, have been central to Ukraine’s success in pushing back Russia’s Black Sea fleet hundreds of miles to open up a shipping corridor for grain and other supplies. Air defense missiles have proven particularly essential in recent weeks as Russia has expanded its attacks on civilian infrastructure this winter. (CNN reported earlier this month that Western intelligence was expecting an expansion of such attacks in an effort by Moscow to break Ukraine’s will to fight.) Shorter-range missiles have enabled Ukrainian forces to defend against Russian tanks and aircraft which greatly outnumber their own.

Assessments of what a Ukrainian defeat would mean for Europe is causing deep fears among some of America’s closest European allies.

“I don’t think people fully realize what Ukraine’s fall would actually mean,” said a European diplomat. “We would see horrible things: ethnic cleansing and total destruction of Ukraine. Remember what they did in Bucha. So, it is already success if we can prevent that from happening. And that is why we must carry on.”

This story has been updated with additional information.