KYIV, UKRAINE -- Russia is throwing more units into its effort to take a key eastern Ukraine city, Western analysts say, after apparent setbacks that have slowed its dayslong onslaught.

The attempt to storm Avdiivka, a heavily defended city that stands in the way of Moscow's ambition of securing control of the entire Donetsk region, is Moscow's most significant offensive operation in Ukraine since the start of the year, the U.K. defence ministry said Tuesday.

The Kremlin's push to claim Avdiivka comes after months of fending off Ukraine's counteroffensive, which Kyiv launched some 16 months after Russia's full-scale invasion.

Both sides are looking for battlefield advantages ahead of winter. They are also looking to secure future weapons supplies for what is shaping up as a protracted war of attrition.

The Kremlin's forces deployed to Avdiivka are likely making slow progress and sustaining high casualties in what is proving a costly effort, the U.K. defence ministry said on X, the social media platform formerly known as Twitter.

The pace of Russia's attack on the heavily fortified target has reportedly slowed in recent days since its launch last week, a Washington-based think tank said.

Citing the Ukrainian General Staff, the Institute for the Study of War said Ukrainian forces pushed back 22 Russian attacks on Monday, down from 30 reported assaults the previous day. Kyiv officials said Russia mounted up to 60 attacks per day in the middle of last week.

The Ukrainian defence ministry said Tuesday that Russia is nevertheless still attempting to encircle the city, with 10 attacks over the previous 24 hours.

The U.K. defence ministry said Russia's effort to overrun Avdiivka "looks increasingly unlikely in the short term."

More widely, Russian forces launched six Shahed drones and one Kh-59 cruise missile over southern regions of Ukraine late Monday, Kyiv authorities said. The Ukraine Air Force of Ukraine claimed to have intercepted all the targets, with no casualties or destruction reported.

Ukraine struck two helicopter airfields near the Russia-controlled Luhansk and Berdyansk regions, the Air Force claimed.

It was not immediately possible to verify the two sides' battlefield claims.