The Guardian and other media outlets said the leaked military and intelligence files suggest that dozens of British special forces personnel have been present in Ukraine so far this year.

The classified US material, which has not been verified, was leaked and then circulated on social media.

The British Embassy in Kyiv said in June 2021 that UK and US special forces were holding "joint training activity" with their Ukrainian counterparts.

But London has not disclosed their continued presence inside the country since Russia's invasion in February last year.

"The widely reported leak of alleged classified US information has demonstrated a serious level of inaccuracy," the UK's MoD said in a statement posted Wednesday on Twitter.

"Readers should be cautious about taking at face value allegations that have the potential to spread disinformation."

Top US diplomat Antony Blinken and defence chief Lloyd Austin spoke with their Ukrainian counterparts on Tuesday as Washington seeks to reassure its allies following the leak.

The breach -- which has sparked a criminal investigation by the Department of Justice -- includes various classified information about Ukraine's battle against invading Russian forces, as well as secret assessments of US allies.

Many of the documents are no longer available on the sites where they first appeared, and the United States is reportedly working to have them removed.