Prosecutor Aleksandr Kiriyak made the request in court on December 7.
The 20-year-old Chelakh is on trial for the deaths of 14 fellow border guards and a forest ranger in a remote outpost near the Kazakh-Chinese border.
In May, the 15 men were found dead at the outpost, which was destroyed by fire.
After he was found days later, Chelakh initially admitted to killing all 15 people.
He later recanted, saying his confession was obtained under pressure and claiming the post was attacked.
On December 7, Chelakh and his lawyers requested a break in the trial, which began on November 19, saying they need time to prepare his testimony.
The trial was adjourned until December 10.