Russian forces have advanced further in Kursk region curling behind Ukrainian forces as part of a major encirclement operation aimed at forcing thousands of Ukrainian soldiers to either flee or surrender in western Russia.

The Russian advances threaten to encircle thousands of Ukrainian soldiers just as Ukraine prepares for talks with top US diplomats in Saudi Arabia on Tuesday, with US President Donald Trump pressing for a swift end to the war.

Ukrainian troops seized about 1300 square km (500 square miles) of Russia's Kursk region in August in what Kyiv said was an attempt to gain a bargaining chip in future negotiations and to force Russia to shift forces from eastern Ukraine.

But by mid-February, Russia had taken back at least 800 square km (300 square miles) of territory in Kursk and in recent days launched a major paratrooper offensive from multiple directions that threatens to cut off Ukraine's supply lines and potential routes of withdrawal.

The gas pumping station is viewed at Sudzha in 2009. – Reuters

A Russian war blog, Two Majors, said Russian forces had cleared the settlement of Ivashkovsky and Russian units were advancing on the so-called "cauldron" in Kursk from at least seven directions.

Yuri Podolyaka, a Ukrainian-born, pro-Russian military blogger, said he was having trouble keeping up with events as the Russian advance was so swift. He said Ukrainian units were trapped in several pockets in Kursk.

"Over the past four days, Russian troops have cleared as much territory in the Kursk region as they sometimes could not even clear in a couple of months," said a Russian blogger close to the defence ministry who goes by the name Rybar.

"The front has been pierced," said Rybar. 

He added that Russian forces were curling up inside the Ukrainian border to cut off the main roads leading out of Kursk to Ukraine's Sumy region.

The Russian offensive raises a serious conundrum for Ukraine just as the spring thaw turns roads to mud tracks: should it withdraw from Kursk, and if so, can it do so without a disorderly rush to the border under intense Russian fire?

A Ukrainian serviceman patrols a street in the town of Sudzha. – Reuters

Russian forces on Sunday recaptured three more settlements in Kursk after special forces crept for miles through a gas pipeline near the town of Sudzha in an attempt to surprise Ukrainian forces.

Russian advances in 2024 and Trump's upending of US policy on Ukraine and Russia have raised fears among European leaders that Ukraine will lose the war and that Trump is turning his back on Europe.

The United States paused military aid and the sharing of intelligence with Ukraine this month after a meeting between Trump and Ukrainian President Volodymyr Zelensky on February 28 descended into acrimony in front of the world's media.

Zelensky said on Sunday that he had received a report from his Commander-in-Chief Oleksandr Syrskyi about the Kursk operation. He did not reveal further detail.

The General Staff of Ukraine's Armed Forces said in its daily report on Sunday evening that Ukrainian forces repelled 27 attacks by Russian forces along the Kursk frontline on Sunday.