"I see no NATO role in an international reaction to the (Syrian) regime," Rasmussen told reporters in the Danish town of Vejle, daily Politiken reported.
He said the alleged use of chemical weapons was "a terrifying and horrible act. Chemical attacks are a clear violation of international standards -- a crime that can't be ignored."
"It demands an international response, so it doesn't happen again," Rasmussen said.
The NATO secretary general has in the past insisted on the need for a political solution to the Syrian conflict.
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The opposition says more than 1,300 people died when toxic gases were unleashed on Eastern Ghouta and Moadamiyet al-Sham. Doctors Without Borders said 355 people died of "neurotoxic" symptoms in the affected areas.
Rasmussen said he firmly believed the Syrian regime was behind the attack.
"I have no doubt that the regime carried out a chemical attack," he said, adding: "When you look at who has the chemical stocks and the means to use them in an attack, you have to say that it is the regime."