The White House said Friday that more evidence is necessary to confirm the Syrian government used chemical weapons against its people, but if that proves to be true, it would be a "game changer."

President Obama addressed the issue in the Oval Office, where he was meeting with Jordan's King Abdullah II, though he didn't commit to any action yet in Syria.

"I have been very clear publicly, but also privately, that for the Syrian government to utilize chemical weapons on its people crosses a line that will change my calculus on how the United States approaches these issues. This is not an on or off switch. This is an ongoing challenge that all of us have to be concerned about," he said.

Much talk has been made of Obama's suggestion that the use of deadly chemical agents could be the "red line" for the U.S.to intervene in the two-year-old Syrian war.

"I think that in many ways a line's been crossed when we see tens of thousands of innocent people killed by a regime, but the use of chemical weapons and the danger that is poses to the international community, to neighbors of Syria, the potential of chemical weapons to get into the hands of terrorists, all of those things add increased urgency to what is already a significant security problem and humanitarian problem in the region," Obama told reporters.

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Earlier Friday, White House spokesman Jay Carney told reporters that the U.S. continues to investigate evidence that the Syrian government has used chemical weapons in its fight against rebels, but he resisted setting a timetable for possible action.

“It has been assessed by our intelligence community with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," Carney said. "Now we are working to establish credible and corroborated facts to build on this intelligence assessment in order to establish a definitive judgment as to whether or not the president's red line has been crossed.”

When pressed by reporters, Carney made it clear there is no projected plan.``I'm not going to set a timeline, because the facts need to be what drives this investigation, not a deadline,''

Meanwhile, two Syrian officials denied Friday that its government forces had used chemical weapons against opposition forces. A government source told the Associated Press that Syrian President Bashar Assad’s army "did not and will not use chemical weapons even if it had them."

Sharif Shehadeh, a Syrian lawmaker,also said the Syrian army "can win the war with traditional weapons" and has no need for chemical weapons.

Carney said President Obama would consider several paths of action if it’s determined that Syria used chemical weapons.``He retains all options to respond to that, all options,'' Carney said. ``Often when people mention all options are on the table, everyone just talks about military force. It's important to remember that there are options available to a commander in chief in a situation like this that include but are not exclusive to that option.''

Top-ranking lawmakers on both sides of the aisle declared Thursday that the "red line" in Syria had been crossed, calling for "strong" U.S. and international intervention after administration officials revealed that the intelligence community thinks chemical weapons were used.

Sen. Dianne Feinstein, D-Calif., chairwoman of the Senate intelligence committee, and Sen. John McCain, R-Ariz., were among those urging swift action.

McCain, who has long called for more involvement in Syria, voiced concern that the administration would use "caveats" to avoid acting on the new intelligence. He said America's enemies are paying "close attention" to whether the U.S. follows through, as the White House signaled it wanted to see more proof before responding to the new information.

"I worry that the president and the administration will use these caveats as an excuse not to act right away or act at all," McCain told Fox News. "The president clearly stated that it was a red line and that it couldn't be crossed without the United States taking vigorous action."

He called for the U.S. to help establish a no-fly zone and "safe zone" in Syria, as well as provide weapons to the "right people."

Defense Secretary Chuck Hagel first revealed the intelligence assessment, which was detailed in a letter to select members of Congress, while speaking to reporters on a visit to Abu Dhabi. The administration then released those letters, which said U.S. intelligence determined with varying degrees of confidence that "the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin."

Secretary of State John Kerry further confirmed that there were two documented instances of chemical weapons use.

The White House, however, stressed that this was not enough to confirm how the nerve gas was released -- though acknowledged it is "very likely" to have originated with the regime of Bashar Assad -- and pressed the United Nations for a "comprehensive" investigation. The letter from the White House director of the Office of Legislative Affairs to leading members of the Senate Armed Services Committee said the assessment was based in part on "physiological samples."

A White House official also urged caution, invoking the Iraq war as an example of why the administration should be absolutely certain before going forward.

"Given our own history with intelligence assessments, including intelligence assessments related to WMD, it's very important that we are able to establish this with certainty and that we are able to provide information that is airtight ... to underpin all of our decision-making," the official said. "That is, I think, the threshold that is demanded given how serious this issue is."

A senior U.S. defense official told Fox News the Defense Department has been preparing a wide range of contingency plans for military involvement in Syria for the past year. President Obama has seen the plans and is fully aware of those options.

The options, according to this official, range from establishing no-fly zones to creating humanitarian zones to launching strikes on chemical weapons sites, select regime leadership and other targets. The official emphasized that no decisions have been made about whether to further involve the U.S. military in Syria and that there are still many questions that need to be answered first.

A United Nations spokesman said the chemical weapon findings reinforce the need for U.N. officials to "be given the requested swift and unfettered access to Syria that it needs to determine whether chemical weapons have indeed been used."

McCain, speaking to Fox News, said in his view the red line "was crossed."

Feinstein, an important voice on matters of intelligence and security, also said it is "clear" those lines have been crossed and "action must be taken to prevent larger scale use."

Sen. Lindsey Graham, R-S.C., reacting to the reports Thursday, said the "number one" goal should be to "secure the chemical weapons before they fall into the wrong hands."

"I think the red line's been crossed and the question is, now what?" Graham said on Fox News.

Sen. Bob Corker, R-Tenn., also said in a statement the assessment is "deeply troubling and, if correct, means that President Obama's red line has certainly been crossed."

But Sen. Bill Nelson, D-Fla., argued that it is not in the United States' "best interest" to go into Syria. "We cannot be absolutely sure about the extent to which Assad's forces have used chemical weapons, although we know they have them," he said in a statement.

Asked if this crossed a "red line" for the U.S., Hagel likewise said they are still trying to assess.

"It violates every convention of warfare," he said.

The Associated Press contributed to this report.