DUBLIN — Diplomatic efforts to end Syria’s civil war moved forward Thursday with Secretary of State Hillary Clinton joining Russia’s foreign minister and the UN peace envoy to the Arab country for extraordinary three-way talks that suggested Washington and Moscow might finally unite behind a strategy as the Assad regime weakens.

In Washington, Defense Secretary Leon Panetta said intelligence reports raise fears that an increasingly desperate Syrian President Bashar Assad is considering using his chemical weapons arsenal — which the United States and Russia agree is unacceptable.