Russia was throwing "waves" of soldiers towards the embattled Ukrainian city of Avdiivka, suffering massive losses in their attempt to capture strategically important territory on the eastern front lines, Ukrainian soldiers say.

Sitting in an indent -- surrounded on almost three sides by Russian forces -- Avdiivka has become a symbol of a grinding war in which neither side has made a decisive breakthrough in more than a year.

But despite having suffered steady losses in troops and equipment, Russia was showing no signs of abandoning its attempt to capture the former coal hub in Ukraine's industrial east.

"The fields are just littered with corpses," Oleksandr, a deputy of a Ukrainian battalion in the 47th mechanised brigade, told AFP.

"They are trying to exhaust our lines with constant waves of attacks," he said, without providing his full name for security reasons.

The city has been destroyed by relentless Russian artillery and aerial bombardments in some of the most intense fighting of the 21-month war.

But around 1,500 of the 30,000 pre-war residents have remained -- mainly pensioners living in their basements and relying on special food convoys to bring in supplies.

The city briefly fell to Russian-backed separatists in 2014, and Ukraine has spent the last nine years building defences and trenches to protect the city.

Fortifications to the south lie just five kilometres (three miles) north of Donetsk, the Russian-controlled capital of one of four regions Moscow claimed to have annexed from Ukraine.

- 'Zombies' -

Throwing huge numbers of soldiers towards the city marks a change in Russia's tactics in the battle for Avdiivka.

In October, Moscow launched a massive attack to try to encircle Avdiivka with hundreds of armoured vehicles.

"Columns of tanks and armoured personnel carriers were advancing," said a 29-year old drone operator who gave his call-sign as "Trauma".

"But they fell into minefields, were hit by drones and anti-tank missiles."

A Western official said the Russian army lost more than 200 armoured vehicles in that failed attack.

Now Moscow has "switched to infantry tactics," said Oleksandr -- "advancing solely at the expense of human resources."

Russian soldiers typically advance at night, in groups of five to seven fighters, Trauma told AFP.

"Then early in the morning, they launch their attack."

Ukraine responds with a barrage of heavy weaponry -- artillery, mortars, grenades, drones and cannons fired from US-supplied Bradley Fighting Vehicles.

"Some die, others keep on coming. It's like a zombie movie," Trauma added.

In throwing hundreds of men at the city, Russia's latest assault on Avdiivka resembles its campaign for Bakhmut earlier this year.

Some 50 kilometres to the north, that city was eventually captured by Russia in perhaps the bloodiest single battle of the war so far.

At Bakhmut, Russia deployed tens of thousands of men -- mainly convicts recruited from prisons by Yevgeny Prigozhin's Wagner Group -- in "storm brigades" as it tried to overrun Ukrainian positions.

Prigozhin regularly described the city as a "meat grinder," and after its capture said Russia's losses exceeded 20,000.

A similar story is now playing out in Avdiivka, Ukrainian soldiers say.

The Russians "see many corpses, but they don't give up," Oleksandr told AFP.

- 'Half surrounded' -

For Moscow, progress may have been slow and costly, but they have made small gains.

Ukraine's general staff said Thursday it had repelled 30 separate Russian attacks over the last 24 hours.

Russia controls territory to the south and east of the city, while Ukraine has held on to an eight-kilometre wide strip of land running from Avdiivka to the northwest.

Russian forces have also recently closed in on a giant coking plant in the northeast of the city as well as the village of Stepove.

From those positions, they are just two kilometres away from Berdychi -- another small settlement, from where the last Ukrainian-held access road into Avdiivka runs.

"That road is vital. If they cut it, it will complicate entry, evacuation, supplies," said deputy commander Oleksandr.

"And it doesn't mean they'll stop there. They're looking for a chance to take the city."

Although "half surrounded," Ukraine has enough troops to carry on defending Avdiivka, Oleksandr said.

"There is no reason to withdraw and give up the city."

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