Explainer-What's happening in Russia's Kursk region and why does it matter?

By Mark Trevelyan

(Reuters) - Ukrainian forces are under intense pressure in Russia's Kursk region, where they grabbed a chunk of land last August and have held on to it through more than seven months of fierce fighting.

What's happening and why is it important?

WHERE IS KURSK AND WHAT HAPPENED THERE?

Kursk is a part of western Russia that borders the Sumy region of Ukraine. On August 6 last year, Ukraine sprang one of the biggest surprises of the war when its troops smashed across the frontier and captured a piece of territory that it said measured 1,376 sq km (530 sq miles) at its peak and included about 100 towns and villages. Since then, Russian forces and troops from Moscow's ally North Korea have clawed back most of that land. Pro-Moscow war bloggers say Ukrainian troops are at risk of being encircled, something Kyiv's top commander denies.

WHAT DID THE KURSK OFFENSIVE MEAN FOR UKRAINE?

The territory that Ukraine captured was a small fraction of the area that Russia has captured in Ukraine since 2014, which amounts to about a fifth of the country. But the operation provided Ukraine with its biggest gains since late 2022 and delivered a massive morale boost: after 2-1/2 years of fending off Russia's invasion, it had stunned its enemy by launching an invasion of its own. President Volodymyr Zelenskiy spoke of "restoring justice", bringing the war home to ordinary Russians and making a mockery of Putin's attempts to set "red lines" to deter adversaries. Ukraine also hoped the operation would slow Russia's advances in eastern Ukraine by forcing it to divert troops to the defence of Kursk - although this did not happen, and Russia's gains in the east only accelerated. Finally, Zelenskiy saw Kursk as a bargaining chip that he said as recently as last month could potentially be traded for Ukrainian territory under Russian control.

HOW DID RUSSIA REACT, AND HOW DID IT TURN THE TIDE?

The first invasion of Russian soil since World War Two was an embarrassment for Putin, who has placed himself in a historic tradition of militarily successful Russian rulers. He played down the military impact of the incursion, which Russia styled initially as a "terrorist" operation, and refused to be drawn into promising a timeframe for the Ukrainians' expulsion. From late October, North Korean troops began arriving in Kursk region to fight alongside Russia under a mutual defence pact agreed months earlier between Putin and his ally Kim Jong Un. Putin has never acknowledged their role on the battlefield but Ukraine and its allies say the North Koreans have played an active part in fighting and sustained heavy casualties. A sharp worsening of Ukraine's position followed a decision this month by U.S. President Donald Trump to pause military aid and intelligence-sharing with Kyiv, although Russia was already reporting near-daily advances by late February.

IF UKRAINE LOSES ITS FOOTHOLD IN KURSK, WHAT THEN?

Ukraine risks losing all its initial gains from the Kursk operation in terms of territory, morale and bargaining power. It could also suffer heavy casualties if its soldiers are encircled or are forced to withdraw under heavy fire. A defeat in Kursk would intensify pressure on Zelenskiy after a series of heavy blows in the past few weeks, including his disastrous White House meeting with Trump on February 28, and weaken his hand in possible peace negotiations.

(Reporting by Mark Trevelyan in London; Editing by Gareth Jones)