A picture shows destruction in the Al-Sukkari district of the northern Syrian city of Aleppo on April 5, 2013.

Syria has probably used chemical weapons against its own people, US Defense Secretary Chuck Hagel said Thursday.

President Barack Obama has repeatedly said evidence of chemical weapons used in Syria would constitute the crossing of a "red line" for America's involvement in the conflict, and could prompt Washington to "take action" in the region.

Analysts in the US intelligence community have determined with "some degree of varying confidence" that Assad's government used sarin gas against rebels and civilians.

“It violates every convention of warfare,” Hagel said, speaking from Abu Dhabi, without specifying the possible quantity of chemicals used or any resulting casualties.

The White House sent a letter to Congress Thursday informing lawmakers about the evidence of chemical weapons. The text shows the White House is holding back from confirming the information, however, and says there's not enough evidence to take action.

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"Given the stakes involved, and what we have learned from our own recent experience, intelligence assessments alone are not sufficient — only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making, and strengthen our leadership of the international community," the letter read.

Obama has said multiple times since last August that any use of chemical weapons in Syria would be a "game changer" for the US response to the ongoing violence.

Last month, the president said that the United States "would not tolerate" the use of chemical weapons against the Syrian people.

The letter was sent by director of the White House's office of legislative affairs, Miguel Rodriguez, to Senators Carl Levin and John McCain. Buzzfeed's Rosie Gray made a copy available online: 

The letter included a detail that intelligence community assessments were "based in part on physiological samples."

General Salim Idriss, the chief of staff of the Free Syrian Army, told CNN earlier this week that rebel forces had found traces of chemical weapons through tests. 

“We took some samples of the soil and of blood,"  Idriss said. "The injured people were observed by doctors and the samples were tested and it was very clear that the regime used chemical weapons."

Syria has denied the accusations. The information minister said yesterday that "even if Syria possessed chemical weapons" for "moral and ethical' reasons it would not use them against rebels or Israelis.

However, the UN has said it will verify the samples in order to validate the chemical weapons claims, the Guardian reported

Hayden said the White House is pressing for a UN investigation of the samples. 

"We are also working with our friends and allies, and the Syrian opposition, to procure, share and evaluate additional information associated with reports of the use of chemical weapons so that we can establish the facts," she added.

Ahmad Ramadan, a member of the Syrian National Coalition's executive body, called the US' conclusion an "important step" and told the Associated Press that the US has a "moral duty" to act to prevent the Assad regime from using chemical weapons.

Hagel's statements appear to show the US position is very close Israel's.

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However, Israel appears to want to distance itself from changing and seemingly contradictory reactions from American officials.

Foreign Ministry spokesman Yigal Palmor declined to comment.

Mark Regev, spokesman for Prime Minister Benjamin Netanyahu, was unavailable for comment.

One source told GlobalPost "the US media is going to have to figure out what all of these responses have meant. The US reactions are not an Israeli matter."

An Israeli defense official who wished to remain anonymous told the New York Times that it's “precisely because this is a red line that we have to establish with airtight certainty that this happened.”

However, Israel's head of research for Israel's military intelligence, Brig. Gen. Itai Brun, has aggressively charged Syria with using chemical weapons

“The bar on the United States is higher than on anyone else," he said, "both because of our capabilities and because of our history in Iraq.”

More from GlobalPost: Syria: Reports of chemical weapons use challenge Obama's 'red line'

Noga Tarnopolsky reported from Jerusalem.

This is breaking news. Please check back for updates.

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