The latest deadline of September 2 was abandoned as a UN-supervised audit of all eight million votes has fallen behind schedule, with both candidates still claiming victory in the fraud-tainted vote.
A NATO summit in Britain from September 4-5 is meant to agree on future support for Afghanistan after the 13-year US-led combat mission ends this year.
But NATO members have stressed a new president should be in place before the summit to prove that the country is becoming a functioning state after receiving billions of dollars of military and civilian aid assistance.
"Following all necessary steps, as required by law, the inauguration of the new President should then be possible soon after," it said.
The US had been leading a strong international effort to push for the next president to be inaugurated by September 2 to allow him to attend the summit.
