BEIRUT — Disjointed diplomatic maneuvering on the Syria crisis by top foreign officials Tuesday seemed to underscore the lack of any coherent international effort to end the fighting, even as another residential bombing and an assassination rocked Damascus.

“The situation inside Syria is turning grimmer every day, and the risk is growing that this crisis could explode outward into an already volatile region,’’ Jeffrey D. Feltman, the United Nations undersecretary general for political affairs, told a meeting of the Security Council.