Syria's main opposition group accused the government of "massacring" more than 1,300 people in chemical weapons attacks near Damascus on Wednesday, prompting demands for an immediate UN inquiry.
German regional daily Stuttgarter Zeitung quoted Merkel, on a campaign swing Wednesday, as saying: "If these data are verified, it is a frightful crime."
During a public debate she also called for the United Nations' mandate to be broadened and for cooperation by the Syrian regime, the daily said.
The accusations, which were strongly denied by Damascus, came as a team of UN inspectors was in Syria to probe previous allegations of chemical weapons strikes levelled against both sides during the 29-month conflict.
Western governments demanded immediate access for the inspectors to investigate the new allegations.