"Spoke to Thiru M.K. Stalin and Kanimozhi. Enquired about the health of Karunanidhiji and offered any assistance required. I pray for his quick recovery and good health," Modi tweeted.
The Prime Minister's remarks came a day after it was revealed that the 94-year-old veteran leader was suffering from urinary tract infection and is being treated at home.
Modi is currently on his five-day, three-nation tour of Africa.
The Kauvery Hospital which is monitoring his health said in a statement on Thursday that there has been a slight decline in Karunanidhi's health due to age-related ailments.
"He is currently being treated... with intravenous antibiotics and fluids. He is being monitored and treated round the clock by a team of medical and nursing professionals, who are providing hospital-level care at his home," Kauvery Hospital's Executive Director Aravindan Selvaraj said.