This map shows how little ground Russian forces gained over 5 months at a cost of 100,000 dead and wounded

• Between December and May, Russian troops managed to capture only several hundred square miles of territory in Ukraine.
• Moscow has paid an enormous price in blood for these limited gains.

A Russian offensive push that began late last year but largely fizzled a few months into the new year achieved only limited gains, which came at a steep price in blood for the forces fighting the Kremlin's war in Ukraine.

Russia has been waging war in Ukraine for nearly 15 months. Intense and brutal fighting along an increasingly static front line has left cities in ruins and resulted in mounting death tolls on both sides, though Russian losses in particular are substantial.

During the five-month period between December 2, 2022 and May 2, 2023, Russian forces captured about 870 square kilometers (336 square miles) of territory in eastern Ukraine, according to estimates provided to Insider by the Institute for the Study of War, a Washington-based think tank that has closely tracked developments in the war. That's roughly 0.1% of the country.

During this time period Russian forces, both regular troops and mercenaries augmented by convicts, suffered over 100,000 casualties, a top White House official revealed during an early May briefing. This astonishing figure includes 20,000 soldiers killed and another 80,000 wounded, National Security Council spokesperson John Kirby told reporters, citing unspecified but declassified US intelligence.

The figure underscores how Russian President Vladimir Putin's war has often attempted to overwhelm Ukraine's defenders with masses of poorly trained and expendable foot soldiers sent forward in human wave attacks.

In November 2022, before Kirby's window starts, Chairman of the Joint Chiefs of Staff Gen. Mark Milley estimated that Moscow had suffered over 100,000 causalities since it invaded in February of that year. It only took Ukraine five months to deal the damage it did in the months prior and double the number of casualties for the invading army.

Kirby also noted in early May that half of the dead Russian troops are mercenaries and ex-prisoners from the notorious Wagner Group paramilitary organization, which has played a key role in fighting around the war-torn city of Bakhmut — the longest and bloodiest battle of Moscow's campaign. Though Kyiv disputes some of Moscow's claims, the city appears to have largely fallen to Wagner forces over the weekend as the Ukrainian troops fell back to defenses along the outskirts.

Kremlin-backed forces — which include Russia's regular military and Wagner — spent months making incremental gains in and around Bakhmut. On Sunday, these forces claimed to have fully seized the obliterated city, although their purported victory has been disputed by Ukraine, which has made some advances along the city's flanks.

Bakhmut is one of two cities that Ukraine's General Staff of the Armed Forces currently assesses to be major focal points in the fighting, with the other being the Donetsk suburb of Marinka. In a Monday Facebook update, the General Staff wrote that Russia is focusing its main effort on areas around those two cities, with other efforts near Avdiivka, Lyman, and Kupyansk.

Ukraine, meanwhile, is in the final stages of preparing for a much-anticipated counteroffensive that is aimed at liberating occupied territory, likely in the occupied eastern and southern regions. It's unclear when, exactly, the counteroffensive might start, but military experts say to look out for the integration of Western heavy armor — like tanks and infantry fighting vehicles — that have been committed by the US and its NATO allies.

If Ukraine's counteroffensive is successful, Kyiv might enjoy reversing some of the limited gains that Russian forces made during the five-month-stretch earlier this year. But even with advanced weaponry from Ukraine's Western partners, there are questions about how much Ukraine can realistically achieve. Even some senior Ukrainian officials have at times expressed reservations about overselling the possibilities of the much-anticipated counteroffensive.

Ryan Pickrell contributed to this report.