"He is still refusing food and medication, but taking water, and seriously jeopardising his health."
Seselj went on hunger strike after the court decided to assign him a defence lawyer against his wishes.
Serbian media reports he has now lost 19kg.
He was transferred to a prison hospital for monitoring last week and doctors will check on his health on Monday.
His trial in The Hague was adjourned indefinitely on Friday. The prosecution had been due to start next week.
Tens of thousands of Serbs protested in front of the US embassy in Belgrade on Saturday in defence of Seselj. Some have claimed he will be seen as a martyr if he dies.
The Radical's leader surrendered to the Hague in 2003 to answer charges of war crimes against non-Serbs in the 1990s and plotting crimes with Slobodan Milosevic, the late Serbian president, who died in detention in March just months before his trial was due to end.
Officials are concerned that Seselj may also die without completing his trial.
Seselj pleaded not guilty and routinely disrupted pre-trial proceedings, insulting judges and calling his assigned lawyers "spies" and "actors posing as lawyers".