New Delhi, Aug. 23 -- Two Bengal tiger cubs died at the Delhi Zoo on Friday morning, leaving only two surviving cubs from the litter of six born on August 4. Officials said both survivors are being treated at the zoo hospital - one remains critical while the other is stable and being hand-reared. Zoo authorities said three of the cubs were shifted to the hospital between August 20 and 21 after showing signs of infection. "Two of them died in the early hours of Friday. The third is stable but under intensive care. The carcasses have been sent to IVRI, Bareilly, for post-mortem," said Delhi Zoo director Sanjeet Kumar. He added that another cub admitted on August 15 is being fed milk separately and is recovering well. Veterinary officer Abhijit Bhawal said the infection could be viral or haemoprotozoan. "We are awaiting confirmation, but our focus is on saving the remaining two cubs," he said. The six cubs were born to seven-year-old tigress Aditi, marking the biggest litter at the zoo in two decades. The zoo has been housing tigers since its inauguration on November 1, 1959. In 2010, the Central Zoo Authority (CZA) started a planned conservation breeding programme of 73 critically endangered wild animal species, as part of the National Zoo Policy, 1998. Under it, the Delhi Zoo has been chosen for tiger conservation and breeding....