President Barack Obama said it was only a matter of time before Syrian President Bashar Assad left office, but squarely opposed a call to launch US military action to force him out.
Obama said what was happening in Syria was "heartbreaking and outrageous", and witness accounts of the devastation after government troops bombarded the rebel stronghold of Baba Amr into submission have given attempts to reach a diplomatic solution renewed urgency.
"The atrocities we saw were beyond our imagination," said one former resident, speaking from a secret location.
UN humanitarian chief Valerie Amos and the joint UN-Arab League envoy Kofi Annan are due to visit Syria this week to see first hand the effects of a conflict that the United Nations says has killed more than 7,500 civilians.
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