The US has warned that Russia could seize Ukraine's key eastern town of Avdiivka - the scene of some of the fiercest fighting in recent months.

"Avdiivka is at risk of falling into Russian control," National Security Council spokesman John Kirby said, citing Ukraine's ammunition shortages.

Ukrainian President Volodymyr Zelensky vowed to do everything to "save as many Ukrainian lives as possible".

Russian troops have made gains in Avdiivka, threatening to encircle it.

The town - which has been almost completely destroyed - is seen as a gateway to nearby Donetsk, the regional Ukrainian capital seized by Russian-backed fighters in 2014 and later illegitimately annexed by Moscow.

Russian President Vladimir Putin launched a full-scale invasion of Ukraine on 24 February 2022.

"Russia is sending wave after wave of conscript forces to attack Ukrainian positions," he said.

"And because Congress has yet to pass the supplemental bill, we have not been able to provide Ukraine with the artillery shells that they desperately need to disrupt these Russian assaults.

"Russian forces are now reaching Ukrainian trenches in Avdiivka, and they're beginning to overwhelm Ukrainian defences."

Earlier this week, the US Senate approved a $95bn (£75bn) foreign aid package - including $60bn for Ukraine - after months of political wrangling, but it faces an uphill battle in the House of Representatives.

Ukraine is critically dependent on weapons supplies from the US and other Western allies to be able to continue fighting Russia - a much bigger military force with an abundance of artillery ammunition.

Nato Secretary General Jen Stoltenberg warned on Thursday that the US failure to approve continued military assistance to Ukraine was already having an impact on the battlefield.

In his video address late on Thursday, President Zelensky said: "We are doing everything we can to ensure that our warriors have enough managerial and technological capabilities to save as many Ukrainian lives as possible."

On Friday, Mr Zelensky is visiting Berlin and Paris where he is expected to sign security pacts with German Chancellor Olaf Scholz and French President Emmanuel Macron.

A similar agreement on security co-operation was signed between Ukraine and the UK in January.

Late on Thursday, Ukrainian General Oleksandr Tarnavsky admitted that "fierce battles" were taking place "within" Avdiivka.

"We value every piece of Ukrainian land, but the highest value and priority for us is the preservation of the life of a Ukrainian soldier," he said.

In its update on Friday, the military general staff said "the planned strengthening of units" was being carried out, as well as "a troop manoeuvre in directions that are under threat".

Some Ukrainian soldiers have privately admitted the town could fall at any moment.

"We're upset," Ukrainian officer Oleksii, from Ukraine's 110th Mechanised Brigade in the Avdiivka area, told the BBC earlier this week, standing beside a huge mobile artillery piece as Russian guns boomed in the distance.

"Currently we have two shells, but we have no [explosive] charges for them… so we can't fire them. As of now, we have run out of shells," said Oleksii. He suggested that the shortages were widespread and having a dramatic impact on the fighting in Avdiivka.

"We feel a very strong responsibility for our guys fighting right now in the town, armed only with assault rifles."

Ukraine's newly appointed commander-in-chief, Oleksandr Syrskyi, visited the frontline in the Avdiivka area this week, acknowledging that the situation there was "difficult".

He said the Russian military did not "count losses", using its troops as cannon fodder.

Kyiv says an elite Ukrainian brigade has now been sent to Avdiivka and reserve artillery has been deployed.

In unverified reports, Russian military bloggers said on Thursday that a key Ukrainian defence position in southern Avdiivka - known as Zenit - was now under Moscow's control.