The Assad regime and Syrian rebels traded accusations Tuesday of launching a chemical attack -- in an escalation that, if confirmed, would mark the first known use of chemical weapons in the civil war and could pressure the U.S. to consider military action after two years on the sidelines.

President Obama has long said the use of chemical weapons would be a "red line" for his administration. However, the warning has always been directed at the Assad regime -- it's unclear whether the administration would feel compelled to intervene if the opposition used chemical weapons. U.S. officials downplayed the allegations Tuesday and said they could find no evidence of chemical weapons, as they uniformly voiced skepticism at the Assad regime's claims.

"We are deeply skeptical of a regime that has lost all credibility," White House Press Secretary Jay Carney said Tuesday.

The regime, whose allegation was backed by ally Russia, said 31 people were killed, including 21 civilians and 10 soldiers, in the attack. Information Minister Omran al-Zoubi called it the "first act" of the newly announced opposition interim government.

Rebels quickly denied the report and accused regime forces of firing the chemical weapon.

More On This... At least 26 reportedly killed in alleged Syrian chemical weapons attack

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The head of Syria's main opposition group, the Syrian National Council, said the group was still investigating the alleged chemical attack near Aleppo.

"Everyone who used it, we are against him, whatever he is," Mouaz al-Khatib told reporters in English in Istanbul. "We are against killing civilians using chemical weapons, but let us wait some time to have accurate information."

The reports could not be independently verified because of tight media restrictions, particularly in government-controlled areas that are virtually shut to all foreign media and outside observers.

But if either version of events is true, it would mark a serious escalation in the two-year conflict. Obama administration officials were pressed repeatedly Tuesday on whether the use of chemical weapons by either side would compel any action by the U.S., which has kept its distance from the civil war.

Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C., seized on the reports to challenge the administration on whether it would follow through on prior warnings.

"If today's reports are substantiated, the President's red line has been crossed, and we would urge him to take immediate action to impose the consequences he has promised," they said in a statement. "That should include the provision of arms to vetted Syrian opposition groups, targeted strikes against Assad's aircraft and SCUD missile batteries on the ground, and the establishment of safe zones inside Syria to protect civilians and opposition groups."

Their Democratic colleague, Sen. Carl Levin of Michigan, also spoke of "ratcheting up of military effort" in response to a potential chemical attack.

"That would include going after some of Syria's air defenses," Levin told Foreign Policy's The Cable, adding that establishing a no-fly zone "would put additional pressure on Assad and also create a zone where Syrian people who are looking for protection and safety could come without crossing the border and becoming refugees."

State Department spokeswoman Victoria Nuland reiterated that the use of chemical weapons would "constitute a red line for the United States," though she made that remark in reference to the Assad regime.

She and Carney, as well as the Pentagon, said they could not confirm the allegations.

"We've seen reports from the Assad regime alleging that the opposition has been responsible for use. Let me just say that we have no reason to believe these allegations represent anything more than the regime's continued attempts to discredit the legitimate opposition and distract from its own atrocities committed against the Syrian people," Nuland said.

"We don't have any evidence to substantiate the regime's charge that the opposition even has CW (chemical weapons) capability," she added.

Carney suggested that the Assad regime could be using the charges to cover up its own use of chemical weapons.

"It is important as fighting in Syria intensifies and the regime becomes more desperate, that the United States and the international community make absolutely clear to Assad that the use of chemical weapons would be totally unacceptable," Carney said. "The president was clear when he said that if Assad and those under his command make the mistake of using chemical weapons or fail to meet their obligations to secure them, then there will be consequences and they will be held accountable."

Amid questions about whether the "red line" had indeed been crossed, the top U.S. military commander in Europe said Tuesday that NATO is conducting contingency planning for possible military involvement in Syria and American forces would be prepared if called upon by the United Nations and member countries.

Adm. James Stavridis, commander of U.S. European Command, told a Senate panel that the United States is "looking at a variety of operations."

"We are prepared if called upon to be engaged," Stavridis told the Senate Armed Services Committee.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said a rocket attack on Khan al-Assal killed at least 26 people but its director, Rami Abdul-Rahman, said he had no information on chemical weapons being involved in the attack.

He said the rocket landed near a military installation in the village.

Syria's policy has been not to confirm or deny if it has chemical weapons. But in July, then-Foreign Ministry spokesman Jihad Makdissi told a news conference that Syria would only use chemical or biological weapons in case of foreign attack, not against its own people.

The Associated Press contributed to this report.