WASHINGTON (MCT) — The Obama administration on Wednesday brought a new round of sanctions against Syrian companies and officials, including the man President Bashar Assad chose as his government’s negotiator in a United Nations peace plan that now appears defunct.

The timing of the announcement — on a day when rebel forces took credit for killing Assad’s top military officials in a brazen bombing in Damascus — only underscored the grim outlook for U.N. special envoy Kofi Annan’s plan for a negotiated, peaceful end to a conflict that’s raged for 16 months.