The Syrian government and the rebels fighting to overthrow President Bashar al-Assad accused each other on Tuesday of firing chemical weapons for the first time in the two-year conflict, which has already cost an estimated 70,000 lives. The missile attack in war-torn Aleppo province reportedly killed at least 25 people, although the Obama administration said it could not confirm that it involved a chemical weapon. If it did, the strike would mark a dangerous escalation in the war, as Syria reportedly has a massive stockpile of deadly gases. President Obama has warned Syria's embattled president, Bashar al-Assad, that deploying chemical weapons would be "totally unacceptable."

Did anyone really cross this dangerous red line? The regime insists the rebels really did, calling it a "grave escalation." Opposition leaders say the victims died of "asphyxia and poisoning after a SCUD missile fired from Damascus struck." A Reuters photographer said people had been admitted to a nearby hospital with breathing problems, saying that "people were suffocating in the streets" in air smelling strongly of chlorine. But a chemical weapons expert tells that video from Aleppo hospitals shows patients with symptoms that were "not really those that are identified with nerve agents or mustard gas, which are the ones most likely to be used" in Syria. And while mustard gas, a throwback to World War I, can emit a chlorine smell, the expert said, so can conventional explosives.

The uncertainty could create a "grave dilemma" for the U.S. and other Western governments that back the rebels, says Damien McElroy in Britain's Telegraph. Obama has made it clear, McElroy says, "that use of chemical weapons by the regime is a 'red line' that would trigger outside intervention." But the attack occurred on the 10th anniversary of the Iraq invasion, which the U.S. justified as necessary to stop Saddam Hussein from using weapons of mass destruction that did not materialize. "The evidence must be clear cut and unambiguous before any action is ordered" in Syria because foreign intervention without "convincing proof that the regime — not the rebels — was responsible would divide the world."

At the same time no western government could easily sit on its hands if public suddenly shifts to back demands to save the Syrian people from their own government. [Telegraph]

The world will have to sit tight until it's clear what happened, though, says Jonathan Marcus at BBC News. Not only is there no proof that either side in Syria is telling the truth in this case, but nobody outside the regime really knows the "exact status of Syria's arsenal, which includes blister agents like mustard and persistent nerve agents like Sarin."

There have been periodic "scares" when Western intelligence agencies claimed to see activity at weapons depots, but so far there has been no hard evidence that chemical warheads have been delivered to the units that might fire them. [BBC News]

If either side's version of events is confirmed, it would be the realization of "one of the nightmare scenarios for this conflict," say Albert Aji and Zeina Karam at The Associated Press. "One of the international community's top concerns since fighting began is that Syria's vast arsenal of chemical weapons could be used by one side or the other or could fall into the hands of foreign jihadi fighters among the rebels or the Lebanese militant group Hezbollah, which is allied with the regime." Assad's government, however, has never accused the rebels — or terrorists, as it calls them — of snatching any chemical weapons, says analyst Mustafa Alani of the Gulf Research center in Geneva, "so we assume that the opposition does not possess such weapons." However, "I would not rule out that the military would use chemical weapons and try to pin it on the rebels," Alani said. "The only strategy that this regime has been left with is character assassination of the opposition and blame the rebels for all the bad things that are happening in the country."