The United States knew that an Al-Qaeda-linked rebel group in Syria was capable of producing sarin gas but ignored it in blaming the Syrian regime for a chemical attack in August, a veteran US journalist has charged.

In a long article published by the London Review of Books, investigative reporter Seymour Hersh accused President Barack Obama's administration of "deliberate manipulation of intelligence" in the Syrian chemical weapons affair to justify intervention.

Administration officials denied the charges and said there was no evidence to support Hersh's claims.

"The suggestion that there was an effort to suppress intelligence is simply false," said Shawn Turner, spokesman for the Office of the Director of National Intelligence.

Hersh does not absolve the Syrian regime of President Bashar al-Assad of responsibility for the August 21 attack in a Damascus suburb, which by US estimates killed more than 1,400 people,

But he contends the administration "cherry-picked" some of the intelligence about it or was silent about other reports that didn't fit with its version of events.

Hersh accuses Obama of omitting "important intelligence" and presenting "assumptions as facts" in a September 10 speech accusing the Assad regime of carrying out the attack.

"Most significant, he failed to acknowledge something known to the US intelligence community: that the Syrian army is not the only party in the country's civil war with access to sarin," Hersh wrote.

"In the months before the attack, the American intelligence agencies produced a series of highly classified reports, culminating in a formal Operations Order -- a planning document that precedes a ground invasion -- citing evidence that the Al-Nusra Front, a jihadi group affiliated with Al-Qaeda, had mastered the mechanics of creating sarin and was capable of manufacturing it in quantity.

"When the attack occurred Al-Nusra should have been a suspect, but the administration cherry-picked intelligence to justify a strike against Assad."

Hersh evokes a top-secret four-page report sent to a senior Defense Intelligence Agency official on June 20 confirming previous reports that Al-Nusra had the ability to acquire and use sarin, thanks to a former Iraqi military chemical weapons expert, Ziyad Tarik Ahmed.

Turner, the ODNI spokesman, responded: "The intelligence clearly indicated that the Assad regime and only the Assad regime could have been responsible for the 21 August chemical weapons attack.

"There's no evidence to support Mr. Hersh's claims to the contrary."