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Bush admin's Iraq WMD claims hang over Syria chemical weapons debate

White House officials strongly suggested Thursday that Bashar al-Assad's regime has used chemical weapons against rebels because of a nerve agent found in victims near Aleppo.

The specter of the bogus claims that former Iraqi dictator Saddam Hussein was hiding weapons of mass destruction — used to justify war —  hangs over the debate on how world leaders will respond to the possibility that Syria deployed chemical weapons.

Obama administration officials say they know they have to deal with the Iraq WMD legacy and will need definitive proof to persuade Russia, Syria’s only remaining ally in the U.N. Security Council, that Bashir Assad’s regime used deadly sarin gas against the opposition in the country’s bloody two-year civil war.

One senior U.S. defense official told reporters Thursday, "We have seen very bad movies before" — referring to previous instances where initial intelligence was proven wrong.

President Barack Obama has called the use of chemical weapons by Assad, a "red line" that if crossed would be a "game-changer" in the U.S. response to Syrian aggression.

The White House said Thursday that the U.S. believes "with some degree of varying confidence" that the Syrian government has used chemical weapons — specifically the nerve agent sarin — against its own people.

A letter from the White House to members of Congress said the assessment was based on "physiological samples" but called for a United Nations probe to corroborate it and nail down when and how they were used.

"We are continuing to do further work to establish a definitive judgment as to whether or not the red line has been crossed and to inform our decision-making about what we'll do next," a White House official said.

The White House said Thursday that the U.S. believes the Assad regime has used chemical weapons, an act that President Obama has previously said would be crossing a "red line." NBC's Jim Miklaszewski reports.

"All options are on the table in terms of our response," the official added.

U.S. intelligence agencies say that blood samples from two attacks last month in Aleppo tested positive for sarin.

Still, those sources say there is “no absolute proof” deadly agents were deployed by Assad's troops.

Administration sources tell NBC News they still have not been able to connect all the dots to prove who actually used the chemical weapons, whom they used them against, or when or where they were used. 

Secretary of State John Kerry discussed Syria with his Russian counterpart in Brussels last week, but the Russians remain unpersuaded to take action against the Syrian government, and the international community is demanding hard evidence to prove Syria is using chemical agents.

The proof, however, could be difficult to obtain.

A spokesman for UN Secretary-General Ban Ki-moon, who said that the United Nations can't take action based on intelligence from one country, said a team of experts assembled to investigate chemical weapons in Syria remains "grounded" in Cyprus because the Assad regime has blocked it from entering the country.

After two years of Syria's bloody civil war, the Obama administration inched ever so slightly toward U.S. military intervention on Thursday. NBC's Andrea Mitchell reports.

The U.N. has repeatedly called on Syria to let its inspection team in.

"The fact-finding team is on standby and ready to deploy in 24-48 hours," the U.N. spokesman said. 

Syria's information minister, Omran al-Zoubi, said in an interview with Russian TV that the government has not and will not use chemical weapons and blamed potential evidence of their existence on "armed terrorist groups," the state news agency reported.

The chemical weapons investigation and counterclaims recall the experience in Iraq, where U.N. inspection teams were hampered in their effort to find weapons of mass destruction amid U.S. intelligence reports suggesting they were being hidden by Saddam.

It was the alleged existence of the so-called WMD the George W. Bush administration used to justify war in Iraq.

Despite a massive search by U.S. forces, no weapons of mass destruction ever turned up.

Sen. John McCain, R.-Ariz.,  was swift to react to the latest reports that Syria used chemical weapons, saying, “I think it's pretty obvious that red line has been crossed." He said the administration should now consider a military approach in Syria he has been advocating for two years that falls short of boots on the ground.

“That is to provide a safe area for the opposition to operate and  to establish a no-fly zone and provide weapons to the people in the resistance who we trust,” McCain said.

A White House official called for a high level of scrutiny, but also caution.

"Given our own history with intelligence assessments, including intelligence assessments related to weapons of mass destruction, it's very important that we are able to establish this with certainty and that we are able to present information in a way that is airtight," the official said.

Reuters contributed to this report.

Related: White House: US believes Syrian regime used chemical weapons