While state-controlled media in Syria are claiming that opposition forces are responsible for what may have been a chemical weapon attack Tuesday in the city of Aleppo, rebel spokesman Qassim Saadeddine is telling Reuters that the opposition was "not behind this attack."

As often happens when news such as this breaks, it's not possible at this point to confirm just what — if anything — has happened. Syrian state media are claiming that at least 15 people were killed in Aleppo by some sort of rocket. The watchdog Syrian Observatory for Human Rights is telling Reuters that as many as 26 people were killed.

American officials have said that President Bashar Assad's regime has prepared chemical weapons for use, and President Obama has warned the Syrian leader that "if you [Assad] make the tragic mistake of using these weapons, there will be consequences and you will be held accountable."

We'll watch for developments.

Update at 12:15 p.m. ET. No Evidence The Weapons Have Been Used, White House Says:

The Associated Press reports that "the Obama administration has no evidence to back up a claim by Syrian President Bashar Assad's regime that the U.S.-backed Syrian rebels used chemical weapons, the White House said Tuesday. A U.S. official went further and said there was no evidence either side had used such weapons Tuesday in an attack in northern Syria, disputing a competing claim by rebels that it was regime forces who fired the chemical weapon."

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Source: NPR