U.S. intelligence has concluded "with some degree of varying confidence" that the Syrian government has twice used chemical weapons in its fierce civil war, the White House and other top administration officials said Thursday.

However, officials also said more definitive proof was needed and the U.S. was not ready to escalate its involvement in the troubled country. That response appeared to be an effort to bide time, given U.S. President Barack Obama’s repeated public assertions that Syria’s use of chemical weapons, or the transfer of its stockpiles to a terrorist group, would cross a "red line."

The White House disclosed the new intelligence Thursday in letters to two senators, and Defence Secretary Chuck Hagel, travelling in Abu Dhabi, also discussed it with reporters.

Reaction in Ottawa

The U.S. assertion that Assad's regime has used chemical weapons triggered a response on Parliament Hill on Thursday afternoon.

"We are very concerned with these reports and remain in close contact with our allies," said a statement from Joseph Lavoie, a spokesman for Foreign Affairs Minister John Baird.

"We have been consistent and very clear — the international community will not tolerate the use of chemical weapons by

[President Bashar] Assad on the Syrian people.

"Ultimately, Assad and his supporters will be held accountable."

"Our intelligence community does assess, with varying degrees of confidence, that the Syrian regime has used chemical weapons on a small scale in Syria, specifically, the chemical agent sarin," the White House said in its letter, which was signed by Obama’s legislative director, Miguel Rodriguez.

Shortly after the letters was made public, U.S. Secretary of State John Kerry said on Capitol Hill that there were two instances of chemical weapons use.

Hagel said the use of chemical weapons "violates every convention of warfare."

It was not immediately clear what quantity of weapons might have been used, or when or what casualties might have resulted.

Ahmad Ramadan, a member of the Syrian National Coalition opposition group’s executive body, called the U.S. assertion an "important step" that should be followed by actual measures taken by the UN Security Council and Friends of Syria group.

Ramadan told The Associated Press by phone from Istanbul that the U.S. has a "moral duty" to act to prevent President Bashar al-Assad from using more chemical weapons.

'Game-changer'

Obama has said the use of chemical weapons would be a "game-changer" in the U.S. position on intervening in the Syrian civil war, and the letter to Congress reiterated that the use or transfer of such weapons in Syria was a "red line for the United States." However, the letter also suggested a broad U.S. response was not imminent.

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A boy, affected in what the government said was a chemical weapons attack, is treated at a hospital in the Syrian city of Aleppo on March 19, 2013. (George Ourfalian/Reuters)

Rodriguez wrote that "because the president takes this issue so seriously, we have an obligation to fully investigate any and all evidence of chemical weapons use within Syria."

The letter went to Republican Senator John McCain and Democratic Senator Carl Levin.

The assessment, Rodriguez said, was based in part on "physiological samples."

The letter also said the U.S. believes the use of chemical weapons "originated with the Assad regime." That is consistent with the Obama administration’s assertion that the Syrian opposition does not have access to the country’s stockpiles.

A senior defence official cautioned that the White House letter was not an "automatic trigger" for policy decisions on the use of military force. The official alluded to past instances of policy decisions that were based on what turned out to be flawed intelligence, such as the Bush administration’s decision to invade Iraq after concluding that Saddam Hussein had weapons of mass destruction, including nuclear weapons.

The official commented only on condition of anonymity because he was not authorized to speak publicly.

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McCain quoted from the letter in making his own comments to reporters on Capitol Hill.

"We just received a letter from the president in response to our question about whether Assad had used chemical weapons," McCain said following a closed briefing with Kerry on Syria and North Korea.

U.S. commanders have laid out a range of possible options for military involvement in Syria. They have also made it clear that any U.S. action would likely be either with NATO backing or with a coalition of nations.

The military options could include establishing a no-fly zone or a secured area within Syria, launching airstrikes by drones and fighter jets, and sending in ground forces to secure chemical weapons supplies.

While Obama has called that a "red line" for taking some kind of further action to assist the rebels, administration officials say the intelligence is not solid enough to warrant such a move.