BEIRUT — Britain called on the United States and other allies Wednesday to do more to shape the Syrian opposition into a coherent force, saying the reelection of President Obama is an opportunity for the world to take stronger action to end the deadlocked civil war.

Also Wednesday, Turkey said NATO members, including the United States, have discussed using Patriot missiles along the Syrian border. It was unclear whether the purpose was to protect a safe zone inside Syria or to protect Turkey from Syrian regime attacks.