According to the US, some of the casualties suffered by Russia near Avdiivka were "on the orders of their own leaders".

Russian and Ukrainian troops have been locked into a fierce battle for the frontline town since mid-October.

Russia is thought to have suffered "significant" losses in this time.

Ukrainian estimates put the number of Russian casualties in Avdiivka at 5,000, while the US says that Russia lost "at least" 125 armoured vehicles and more than a battalion's worth of equipment.

A Ukrainian army spokesperson said that Russian troops were refusing to attack Ukrainian positions near Avdiivka because of heavy losses and that there had been mutinies in some units.

"Russia's mobilised forces remain under-trained, under-equipped and unprepared for combat, as was the case during their failed winter offensive last year," National Security Council spokesman John Kirby said in a briefing on Thursday.

He said that the Russian military "appears to be using what we would call 'human wave' tactics, just throwing masses of these poorly trained soldiers right into the fight."

"No proper equipment, no leadership, no resourcing, no support. It is unsurprising that Russian forces are suffering from poor morale," Mr Kirby added.

Taking Avdiivka - which lies near the Russian-occupied city of Donetsk - would allow Russian troops to push the front line back, making it harder for the Ukrainian forces to make further advances into Donetsk region.

Avdiivka has been all but abandoned by its 30,000 residents as Russian forces continue to pummel it. Earlier this week, Ukrainian President Volodymyr Zelensky described the situation as "particularly tough".

On Thursday, the US announced a new $150m (£123.7m) military assistance package for Ukraine that includes artillery and small-arms ammunition, as well as anti-tank weapons.

However, future aid to Ukraine is in doubt following the election of Republican Mike Johnson as speaker of the US House of Representatives earlier this week.

Mr Johnson - who is on the right wing of the Republican Party - is against further US aid to Ukraine and has previously supported amendments to block it.

The US is the largest military donor to Ukraine, having spent more than $46bn (£37bn) so far, plus tens of billions more in financial and humanitarian aid.

Russian President Vladimir Putin launched a full-scale invasion of Ukraine in February 2022.

 By Laura Gozzi