Ukraine's military said Thursday it was fighting more than 30,000 rebel soldiers in the east of the country -- including up to 10,000 Russians and mercenaries from as far afield as Brazil.

"Today in the territory of Donbass, there are 32,400 fighters, of which 6,000 to 10,000 are Russian armed forces. All the rest are mercenaries and members of the so-called illegal paramilitary groups," said Ukraine military spokesman Oleksandr Rozmaznin, according to Interfax Ukraine.

He said the army had also detained mercenaries from Israel, Serbia, Spain, Italy and even Brazil.

His figures, which are the most precise issued by Ukraine to date, could not be independently verified.

Rozmaznin claimed Russia is regularly rotating soldiers in and out of Ukraine because they "cannot stand the psychological pressure -- they do not understand what they are fighting for."

Russia has repeatedly denied providing direct military assistance to the rebels in east Ukraine, at times claiming that any Russian soldiers in the region are volunteers on holiday.

The conflict has killed over 4,300 people and displaced a further 930,000 since it began in April, according to the UN.