A senior official from US President Joe Biden's administration, who spoke on the condition of anonymity to discuss the private consultations, told reporters Ukraine was not mobilising or training enough new soldiers for the conflict.
He said the outgoing Democratic administration wants Ukraine to lower the mobilisation age to 18 from the current age of 25 to help expand the pool of fighting-age men.
"The need right now is manpower," he said.
"The Russians are in fact making progress, steady progress, in the east, and they are beginning to push back Ukrainian lines in Kursk ... Mobilisation and more manpower could make a significant difference at this time as we look at the battlefield today."
The much larger Russian military outnumbers forces of the Ukrainian army. (AP PHOTO)
The White House has pushed more than $US56 billion ($A86 billion) in security assistance to Ukraine since the start of Russia's February 2022 invasion and expects to send billions more before Biden leaves office in less than two months.
But with time running out, the Biden White House is also sharpening its viewpoint that Ukraine has the weaponry it needs and now must dramatically increase its troop levels if it is going to stay in the fight with Russia.
The official said the Ukrainians believe they need about 160,000 additional troops but the US administration believes they probably will need more than that.
More than a million Ukrainians are in uniform, including National Guard and other units.
Ukrainian President Volodymyr Zelenskiy has also been hearing concerns from other allies that Ukraine has a troop level problem and not an arms problem, according to European officials who requested anonymity to discuss the sensitive diplomatic conversations.
The European allies have also stressed that the lack of depth means that it may soon become untenable for Ukraine to continue to operate in Russia's Kursk border region that Ukraine seized this year.
The situation in Kursk has become further complicated by the arrival of thousands of North Korean troops who have come to help Russia try to claw back its territory.
The stepped-up push on Ukraine to strengthen its fighting ranks also comes as Ukraine braces for US president-elect Donald Trump to take office on January 20.
The Republican said he would bring about a swift end to the war and has raised uncertainty about whether his administration would continue US military support for Ukraine.
Ukraine has taken steps to broaden the pool of draft-eligible men but the efforts have only scratched the surface against a much larger Russian military.
In April, Ukraine's parliament passed a series of laws, including lowering its draft-eligible age for men from 27 to 25, aimed at broadening the universe of men who could be called on to join the grinding war.
Those laws also did away with some draft exemptions and created an online registry for recruits.
They were expected to add about 50,000 troops, far short of what Zelenskiy said at the time was needed.
Conscription has been a sensitive matter in Ukraine throughout the war.
Russia's own problems with adequate troop levels and planning early in the war prevented Russian forces from taking full advantage of its edge.
But the tide has shifted and the US says the Ukrainian shortage can no longer be overlooked.
Some Ukrainians have expressed worry that further lowering the minimum conscription age and taking more young adults out of the workforce could backfire by further harming the war-ravaged economy.
The US official added that the administration believes that Ukraine can also optimise its current force by more aggressively dealing with soldiers who desert or go absent without leave.
with AP
