Ratko Mladic, also known as the "Butcher of Bosnia," was hospitalized after his health deteriorated in prison in the Netherlands.
According to reports in the Serbian media, the convicted war criminal is in critical condition.
"He was taken to the prison hospital due to general weakness, lung inflammation and heart complaints. Controls continue. We were informed that the results will be known on Monday at the latest," said his son, Darko Mladic.
It was also claimed that Mladic caught COVID-19 at the beginning of August and that the treatment in prison was slow and inadequate.
Mladic executed the most well-documented and widely known genocide and crimes against humanity in Europe after World War II.
The UN court in The Hague sentenced former Bosnian Serb commander Mladic to life in prison for spreading terror among civilians in the capital Sarajevo and other parts of Bosnia and Herzegovina in an attempt to clear non-Serbs from certain territories.
More than 8,000 Bosnian Muslim men and boys were killed when Bosnian Serb forces attacked the eastern town of Srebrenica in July 1995, despite the presence of Dutch peacekeeping troops.
The Serb forces were trying to wrest territory from Bosnian Muslims and Croats to form a state.
The bodies of victims have been found in 570 places across the country.