Hardline Islamic parties in Pakistan say an apology by a US magazine over a story about a desecration of the Koran is a crude bid to ease Muslim anger.
The report has triggered anti-US rallies in the Muslim world
Newsweek said US interrogators at Guantanamo Bay flushed a copy of the Koran down a toilet. It now says it erred in reporting the story.
But White House spokesman Scott McClellan said the decision not to issue a full retraction was "puzzling".
At least 15 people died in riots following the article's publication.
Pakistan has said it wants Washington to investigate the allegations, published last week.
The foreign ministry in Islamabad said it would expect the US to share its findings with the Pakistani government.
As well as at least 15 deaths in Afghanistan, more than 100 people were injured in violent protests across the Muslim world, from Pakistan to Indonesia, after the article's publication.
The head of Pakistan's conservative six-party Islamic alliance, Qazi Hussain Ahmed, told the BBC that Newsweek's clarification held no weight.
"There have been reports by the prisoners who have been released from Guantanamo Bay of desecration of the holy Koran, and different atrocities perpetrated on them. Therefore, the clarification of Newsweek has no meaning."
The apology would not stop his alliance from holding more protest rallies on the issue, he added.
A spokesman for Afghan president Hamid Karzai, Jawed Ludin, welcomed Newsweek's statement but said it disapproved in the strongest terms of Newsweek's approach to reporting, "which allowed them to run the story without proper examination beforehand".
The Pentagon has said there is no substance to the specific allegation, but a spokesman for the US military in Afghanistan says there will still be a thorough investigation into the claims.
Several claims of desecration of the Koran have been made by former inmates of the US facility in Cuba.
For the original story, Newsweek says that one of its reporters spoke to "his original source, the senior government official, who said that he clearly recalled reading investigative reports about mishandling the Koran, including a toilet incident".
"But the official, still speaking anonymously, could no longer be sure that these concerns had surfaced" in a forthcoming report by the US military, the magazine added.
It apologised for carrying the report.
Mr Whitaker told the Reuters news agency that he no longer knew whether the occurrence was genuine.
Some 520 people remain incarcerated at Guantanamo Bay
"As to whether anything like this happened, we just don't know," he said.
The White House said that Newsweek's response was insufficient.
"While Newsweek now acknowledges that they got the facts wrong, they refuse to retract the story," spokesman Scott McClellan said.
"The report has had serious consequences. People have lost their lives. The image of the United States abroad has been damaged. I just find it puzzling."
Insulting the Koran or the Prophet Muhammad is regarded as blasphemy and punishable by death in both Pakistan and Afghanistan.
The US is holding about 520 inmates at Guantanamo Bay, many of them al-Qaeda and Taleban suspects captured in Pakistan and Afghanistan following the 11 September 2001 attacks in the US and subsequent US-led invasion of Afghanistan.