The Rwandan leader claims that Kinshasa has armed FDLR and uses it to fight the M23, who are Congolese.
"I must confess, President Ruto is supporting Rwanda, and I won't say anything more about that," Tshisekedi said during a DRC session hosted by the Brookings Institution.
The DRC demanded the withdrawal despite UN concerns about rampant violence in the east of the country.
The country currently has 6.7 million people displaced within its borders, amid serious flooding emergencies and a resurgence of measles and cholera outbreaks.
Rwanda cited repeated violations of its airspace by Congolese fighter jets as contributing to the heightened tensions.
This marks SA's first fatalities since it deployed 2,900 troops to the mineral-rich region last December to quell violence.