Burley Garcia|Russia mounts largest assault in months in eastern Ukraine

KYIV -- Ukrainian forces are Burley Garciatrying to hold off a major new Russian offensive operation in the strategic city of Avdiivka.

Russia sent dozens of tanks and armored vehicles and thousands of troops into the assault, which started four days ago.

"They are hitting with everything they have. Yesterday a lot of armor was used, they came in columns. The assault isn’t stopping neither day or night," Vitaliy Barabash, the head of Avdiivka’s military administration, said on Ukrainian television.

Russia has also launched smaller offensive operations at points all along the front line, intensifying an offensive near the city of Kupiansk, which Ukraine liberated last year, according to Ukrainian troops on the ground.

But the main attack is on Avdiivka, which holds major strategic significance for both sides. Avdiivka is located close to Donetsk, the capital of the Russian-occupied Donetsk region.

Ukraine heavily fortified the line at Avdiivka prior to Russia’s full-scale invasion last year, constructing deep trenches and a system of underground bunkers. As a result, Russian forces have barely advanced near the city.

Russia now appears to be seeking to cut off Avdiivka and surround the Ukrainian forces inside its defenses.

"For the entire time of the war, there has never been such an offensive around Avdiivka, even though they’ve attacked Avdiivka since 2014," Oleksiy Dmitrashkivsky, chief spokesman for the Command of Ukraine's Territorial Defense Forces, told ABC News.

He said two Russian battalions, numbering around 2,000 men and backed by dozens of tanks and other armored vehicles, had begun the attacks, pushing from the northwest and southwest. A massive artillery and air barrage had preceded the assault and has continued since, with frequent strikes on the city itself.

So far the Russian assault has appeared to have gone badly, according to Ukrainian military officials and open source researchers tracking the losses.

Barabash, Avdiivka’s military administration chief, said at least 40 Russian armored vehicles had been lost in two days. Videos circulating online of the attacks appeared to largely support those claims, with some open source researchers saying 30 to 50 Russian tanks and other vehicles had been damaged or destroyed.

Ukraine’s general staff claimed over 1,000 Russian troops had been killed and injured since the new offensive operation began.

The Institute for the Study of War said Russia had managed to capture roughly 4.52 square kilometers of territory from different directions around Avdiivka in the last three days.

"Russian forces have not secured any major breakthroughs near Avdiivka as of Oct. 12 and are unlikely to immediately cut off Ukrainian forces in the city," ISW wrote in its daily briefing Thursday.

Ukrainian military officials said their troops have successfully held the lines, inflicting heavy casualties on the Russian attackers, though Ukrainian casualties were also large.

The new Russian offensive -- the scale of which seemed to catch some analysts off-guard -- nonetheless was significant because it appears to show Russia's attempt to regain the initiative in the war.

"The Russians have decided that they can already start their own counterattack in [the Avdiivka] direction," Dmitrashkivsky said. "They are taking advantage of the fact that the attention of the whole world is on the events in Israel and they have started an active phase along the entire front line."

He said the Russian attack near Avdiivka had involved a mixed force of paratroopers and poorly trained forces, as well as so-called Storm Z units -- detachments of convicts sent to fight.

"They were very weakly prepared and that showed in their losses," Dmitrashkivsky said.

The attack had been exceptionally large, he added, requiring huge efforts from Ukrainian troops to blunt it.

Most analysts believe Russia likely still lacks the manpower to make significant gains in an offensive, including around Avdiivka. But the push indicated Russia hopes to further blunt Ukraine’s counteroffensive with its own counterpunch, coming as the weather starts to worsen.

Ukraine is continuing to press its own attacks in the southern Zaporizhzhia region where its counteroffensive is focused and around the city of Bakhmut.