Russian special force troops who took part in a covert operation in Kursk are reported to be showing signs of being severely poisoned. Earlier in March, Russia's army took Ukraine by complete surprise, after 800 special force soldiers crawled 15 kilometres through an unused section of gas pipeline in the Kursk region.

The Russians were then able to mount a sneak attack on Ukrainian units defending Sudzha, which they successfully recaptured in a major blow for Kyiv. Mounting evidence suggests that the aftermath of the operation is having a devastating impact on the health of the troops. Participants reported a sharp deterioration in their health, high fever, cough and chest pains, allegedly leading to mass hospitalisations.

According to the Russian state media outlet RT, soldiers started to contract an unusual and dangerous disease, which doctors had never encountered before.

Medics have been forced to look for unconventional ways to treat the soldiers, as they fight to save their lives.

Timso, a commander from the Chechen Akhmat special forces unit, told RT: "I almost died, everything happened so quickly.

"I started to feel worse in geometric progression, temperature 38-39, cough, chest pain."

A military doctor said that almost all the special force soldiers who took part in the operation developed similar symptoms.

"This is a non-specific inflammation of the lungs after inhalation of galvanised products contained in the pipeline," he explained.

"A rather insidious condition that does not develop immediately, but on the second, third and sometimes fourth day after they leave the pipe."

The medic added that he and his colleagues had never come across such a complex poisoning, that combined several types of toxins.

Ukraine insists its forces still hold foothold inside the Kursk region, despite the recent Russian gains.

Kyiv's army seized the territory after staging a lightning incursion last August, that shook the Kremlin.

Putin has amassed an army of some 60,000 soldiers - including around 12,000 North Koreans - in a bid to drive the Ukrainian out of the Russian border region.