White House officials on Thursday said they believed that Syrian President Bashar al-Assad used chemical weapons against rebel forces, but they did not threaten military action against the leader's regime despite the apparent crossing of President Obama's so-called red line.

Although U.S. intelligence officials believe Assad deployed the nerve agent sarin, the White House's only response was to call for an investigation by the United Nations. The U.S. assessment was based on new evidence that emerged in the past 24 hours, officials said.

Secretary of State John Kerry on Capitol Hill pointed to two separate examples of Assad's forces unleashing chemical weapons.

Administration officials insist that Obama has not wavered from his red line and say that the movement or use of chemical weapons by Assad's regime would lead to a heightened U.S. response. Yet, the White House on Thursday refused to outline any potential action.

"All options are on the table," said a White House official. "We are continuing to do further work to establish a definitive judgment as to whether or not the red line has been crossed."

Obama has been hesitant to intervene in the civil war, which has lasted more than two years and taken more than 60,000 lives, condemning Assad's actions but resisting calls for Americans to directly assist the rebels. And administration officials Thursday alluded to the buildup to the war in Iraq, saying they didn't want to repeat the mistakes that led to the U.S. invasion there.

Some analysts welcomed the White House's measured approach.

"Once you go in, you can't turn back," said Lawrence Korb, assistant defense secretary under President Reagan. "And remember, Bush was 'certain' about WMDs in Iraq. Without a chain of custody [in Syria], you can't be certain."

In the short-term, administration officials are calling for a United Nations investigation of Syrian activity. However, Assad has been unwilling to cooperate with the U.N., limiting the prospects for such an analysis.

Considering the public's aversion to another war, officials on Capitol Hill and in the White House played down any talks of American boots on the ground in Syria. Instead, officials are more likely to explore the possibility of arming rebel forces or establishing a no-fly zone over Syria, a method recently used to bring down Libyan dictator Moammar Gadhafi.

However, some said Obama wasn't moving swiftly enough.

"I am deeply concerned with reports that further confirmation of use may be outsourced to the United Nations," said House Speaker John Boehner, R-Ohio. "If Assad sees any equivocation on the red line, it will embolden his regime."

Even some Democrats shared similar skepticism about the administration's response.

"I am very concerned that with this public acknowledgement, President Assad may calculate he has nothing more to lose, and the likelihood he will further escalate this conflict therefore increases," said Sen. Dianne Feinstein, D-Calif., adding that the red line had clearly been crossed.