Nelson Mandela continues to show signs of improvement after two months of hospital treatment, the South African presidency said Tuesday, but gave no details of his condition after reported scares. \"Former president Nelson Mandela still remains in a critical but stable condition in hospital in Pretoria, and continues to show improvement,\" it said in a statement. The update was issued after US network CBS reported that Mandela underwent a\"surgical procedure to unblock a dialysis tube\" and had two scares in recent weeks. \"On one occasion Nelson Mandela was not responding to his meds (medicines), and on another his life support machine showed him in distress.\" The government refused to comment on the reports. But since being rushed to hospital on June 8, the 95-year-old Nobel peace laureate has at times appeared to be at serious risk. Friends and family said he has been breathing with the aid of a ventilator and had undergone dialysis. President Jacob Zuma urged the public to \"continue praying for Madiba,\" using Mandela\'s clan name, and take part in community projects that will take forward his work. Mandela\'s 95th birthday earlier this month was marked in South Africa by an outpouring of charitable deeds, with many people offering 67 minutes of service for the 67 years he was in public service.