Not long ago, the Bush administration quietly asserted that it might use nuclear weapons in a conflict first if the US suffers a biological or chemical weapons attack.

NPR: Nuclear Weapons Gain Importance for Regional Powers

Anthony Loyd, a journalist for the Times newspaper, told the BBC about the aftermath of one suspected chemical weapons attack earlier this month in the northern city of Aleppo.

BBC: 'Growing evidence' of chemical weapons use in Syria - UK

While Ban told reporters at the United Nations he was aware of the rebel claims of a chemical weapons attack by the government, he did not say whether those claims will also be investigated.

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Syria claimed rebels used chemical weapons in an attack Tuesday in Khan al-Asal in the northern province of Aleppo.

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The Syrian government called for the United Nations to investigate alleged chemical weapons use by rebels in the attack that killed 31 people.

NPR: Lawmakers: Syria Chemical Weapons Could Menace US

Charges that Assad's forces used chemical weapons surfaced months ago, but it was a March 19 attack in Aleppo that drew global attention.

CNN: Obama must act on Syria chemical weapons

In a separate development, UN Secretary General Ban Ki-moon said he was waiting for the Syrian government to allow a team of chemical weapons experts into the country for an expanded investigation following an attack on a village outside Aleppo in March.

BBC: Syria crisis: Bashar al-Assad says West will 'pay price'

Before the attack in Aleppo on Tuesday, Syria's government had never alleged rebels had access to chemical weapons, only refuting rebel claims last year that it had used such weapons or ever would against its own people.

WSJ: Syria Chemical Report Spurs Probes, Theories