Michael Semple told Britain's The Guardian Wednesday the offer is "game changing."
"It's an open question whether the Americans appreciate how momentous a step this was for the Taliban. It is serious and it is profound," Semple said.
A number of European officials and observers said the Obama administration is in danger of missing a historic opportunity for a peace settlement in Afghanistan.
"We know there is a political risk involved in the middle of (U.S. presidential) elections, but we also believe the earlier they do this, the less political risk," one senior European official said.
Several diplomats stressed to The Guardian the Taliban offer represents a dramatic change in policy.
Semple said if both sides implement confidence-building measures such as prison transfers, "it is realistic to think there could be a cease-fire in 2012."
The White House did not respond to a request for comment, the newspaper said.