"I don't know their motivations, other than to say they continue to rain down horrific attacks on their own people," Ventrell told reporters. "So that speaks pretty loudly and clearly."
If the Assad regime is serious, he said, it should inform the U.N. peace envoy, Lakhdar Brahimi of its readiness for talks. Ventrell said the Assad regime hasn't yet done that.
Obama administration officials have debated whether the U.S. should arm the rebels, with Defense Secretary Leon Panetta and Joint Chiefs of Staff Chairman Gen. Martin Dempsey having said they urged such a course of action. The White House has been unwilling to do so for fears the weapons could end up in the wrong hands. Currently, the U.S. provides only non-lethal support and humanitarian aid.
The United Nations says at least 70,000 people have been killed in Syria's 2-year civil war, which began as an uprising against Assad's regime.
Kerry said the Syrian people "deserve better" than the violence currently gripping their country as he stood alongside Britain's Foreign Secretary William Hague.
Associated Press writers Cassandra Vinograd in London and Bradley Klapper in Washington contributed to this report.
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