Animal welfare activists had previously said the zoo was in talks to borrow a Canadian Forces plane to transport the pachyderms to an elephant sanctuary in California.
The Defence Department issued a statement Friday saying it was in talks with the zoo, but that no arrangements could be made on time to move the elephants before warm weather sets in.
Animal rights groups were calling for the three elephants to be relocated before the end of June.
The department says the earliest the animals could be relocated would be the fall, adding there has been no final decision on whether a military aircraft would be available at that time.
The department said it receives many requests for help each year and makes decisions based on logistical and financial considerations.
City councillors voted last November to transfer the only three elephants still living at the Toronto Zoo to the Performing Animal Welfare Society Sanctuary (PAWS) in San Andreas, Calif., by year's end.
The move has been delayed in part due to the logistics of transporting the three huge animals.
Transportation logistics are the latest headaches to plague relocation efforts for the zoo's elephants. City and zoo officials originally clashed over where Toka, Thika and Iringa should be housed due to concerns about tuberculosis at PAWS.
City council accepted an independent infectious disease report from a specialist veterinarian which found that PAWS is a safe facility and meets the requirements of the due diligence process.
U.S. officials peg the relocation costs at up to US$1 million, but PAWS will not be on the hook for the cost.
Animal rights activist and former "The Price is Right"' host Bob Barker has vowed to pick up the tab.