Obama's overall ratings also declined to a new low, with only 41 per cent of Americans saying they approved of his performance, according to the USA Today/Gallup poll - his worst showing since taking office in January 2009.
The percentage of Americans who say the United States made a mistake in sending troops to Afghanistan rose to 43 per cent, compared to 38 per cent before the release of tens of thousands of classified documents on the war.
Public confidence in Obama's war policy has also plunged to 36 per cent, down from 48 per cent in February survey, the poll said.
Even though he has failed to make a sizeable dent in high US unemployment, Obama received a better rating for his management of the economy than for the war, with 39 per cent supporting his handling of the economy, it said.
Growing public doubt about the Afghan war came as the death toll for US troops in July hit a record high of 66.
The leak of 92,000 secret US government documents on the war has provided more ammunition to opponents of the mission, who point to files alleging Pakistan, a US ally, has cultivated links with insurgents in Afghanistan.
US officials have condemned the website Wikileaks for releasing the documents, and the poll showed most Americans agree the papers should not have been posted on the Internet.
The survey said 66 per cent of Americans said it was wrong for the website to have published the files, while 25 percent say Wikileaks was in the right.
Pakistani President Asif Ali Zardari, meanwhile, admitted in an interview with the French newspaper Le Monde that coalition forces "are losing the war against the Taliban" in Afghanistan.
COMMENTS (1)
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ