Updated 6:10 p.m. ET --
U.S. intelligence has determined "with varying degrees of confidence" that Syrian President Bashar al-Assad has used chemical weapons against his own people, pressing the two-year-old civil war up against the "red line" previously established by President Obama.
"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 wrote in a letter to members of Congress Thursday.
Obama has said that the use of chemical weapons in Syria would cross a "red line" and prompt unspecified U.S. action. What Thursday's news could mean in terms of U.S. involvement is not yet known. The bloody Syrian conflict is estimated to have caused more than 70,000 deaths--and created a growing refugee crisis.
Earlier this week, Israeli officials said they believed chemical weapons had been used against the opposition in Syria, following similar assessments by the French and British.
Defense Secretary Chuck Hagel shared the same information with reporters while traveling in Abu Dhabi Thursday. He stressed that U.S. officials are continuing to assess and gather information on the use of the weapons.
"We cannot confirm the origin of these weapons, but we do believe that any use of chemical weapons in Syria would very likely have been originated with the Assad regime," Hagel said. Hagel did not say whether the use of chemical weapons crossed the president's "red line."
"As to a red line, my role as Secretary of Defense is to give the president options on a policy issue, that's a policy issue," he said. "And we'll be prepared to do that as the president requires options."
The White House is pressing for a “comprehensive United Nations investigation that can credibly evaluate the evidence and establish what took place" in Syria and is seeking "credible and corroborated facts" before taking action, according to a White House official.
"It's precisely because we take this red line so seriously that we believe there is an obligation to fully investigate any and all evidence of chemical weapons use within Syria," the official said.
But in a statement Thursday evening, a spokesperson for the United Nations Secretary-General said that the U.N. "is not in a position to comment" on intelligence from one nation.
"The Secretary-General has already put together a technical expert team to conduct a fact-finding mission to look into all serious allegations of the possible use of chemical weapons in Syria," the statement read. The U.N. inspections team is currently on standby in Cyprus, barred from entry into Syria. The statement called on Syrian authorities to grant the team access in order to conduct an investigation.
U.S. intelligence agencies continue to investigate when and where the weapons may have been used. Secretary of State John Kerry pointed to two instances of chemical attacks launched in Syria, during a Thursday summit with reporters. The White House letter does not specify how many times the weapons were allegedly used. According to a White House official, the intelligence assessment is based on physiological samples, and the U.S. is working with "friends and allies as well as the Syrian opposition" to learn more about the use of chemical weapons.
Senator Bob Corker, a Republican from Tennessee, focused on the Syrian opposition while pointing to an oncoming power vacuum after Assad's departure. He called that "second war" within the opposition groups "almost more important" than the civil war.
"That's the one that, candidly, has even greater national interest implications to us and our allies in the region," Corker said on Andrea Mitchell Reports Thursday. "So obviously we're going to need to take additional actions more than what we're taking now, regardless, in my opinion, regardless of whether chemical weapons have been used there."
Corker told Mitchell that he expects "additional activity" by the U.S., but did not speculate as to what that may include.
"It engages the president, because it has to, because he's clearly stated that this is a red line," Corker said of the new intelligence on the use of chemical weapons, acknowledged by the White House in response to a letter from a bipartisan group of senators including Corker. In a statement earlier Thursday, Corker called the intelligence assessment "deeply troubling."
"I'm looking forward to that consultation with Congress, as to how we move forward and take care of our national interests in the region," Corker told Mitchell Thursday. In a previous letter to Obama, Corker insisted that the Senate be consulted before expanding U.S. involvement in Syria.