Russian troops are close to ejecting Ukrainian forces from their foothold 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.
So what's happening and why is it important?
Where is Kursk and what's been happening there?
Kursk is a part of western Russia that borders the Sumy region of Ukraine. On August 6th 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 1,376sq km 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 close to 90 per cent of that land.
President Vladimir Putin visited Kursk on Wednesday in a sign of confidence that Russia is close to recapturing the entire region, and ordered his top commander to finish the job as soon as possible.
Ukraine's top army commander, Oleksandr Syrskyi, said on Wednesday that Kyiv's troops would keep operating in Kursk as long as needed and that fighting continued in and around the town of Sudzha.
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.
Ukraine's 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 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.
Zelenskiy also 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 the second World War 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 the 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.
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.
Withdrawing after months of fierce fighting would be another blow to Zelenskiy, already under severe pressure after his disastrous White House meeting with President Donald Trump on February 28th.
But there is also pressure on Putin, as the United States urges him to join a proposed 30-day ceasefire that Ukraine told Washington on Tuesday it was ready to support.
Putin's surprise visit to Kursk, during which he donned combat fatigues, sent a signal of military resolve to the West and underlined his determination to ensure the ejection of Ukrainian forces before entering any negotiation to end the war.
