The call for Kyiv to boost its ranks came amid speculation that incoming president Donald Trump will take a new approach that could include pushing Kyiv into a peace deal with Moscow. A senior Biden administration official said that Ukraine was facing an "existential" recruitment crunch as it confronts a much larger enemy with more advanced weapons, and with its stocks of volunteers dwindling.

"The simple truth is that Ukraine is not currently mobilizing or training enough soldiers to replace their battlefield losses while keeping pace with Russia's growing military," said the official, speaking on the condition of anonymity. Pressed on what Washington considers an appropriate minimum age, the official replied that "we think there's real value in them considering lowering the recruiting age to 18" -- in line with the US benchmark.

Biden's White House later clarified that it was not talking about making the huge flow of US military aid to Kyiv dependent on a conscription age change.

"We're absolutely going to keep sending Ukraine weapons and equipment. We know that's vital. But so, too, is manpower at this point," National Security Council spokesman John Kirby said in a statement. "In fact, we believe manpower is the most vital need they have. So, we're also ready to ramp up our training capacity if they take appropriate steps to fill out their ranks."

President-elect Trump signaled a possible change of approach with Biden on Wednesday when he nominated the staunch loyalist and retired general Keith Kellogg as his Ukraine envoy. Kellogg has previously suggested that Washington could leverage the huge flow of military aid to Kyiv as a means of pushing it to enter peace talks with Moscow. He has also proposed promising that NATO would not give Kyiv membership for years in exchange for Russia offering security guarantees.