Western diplomatic sources and analysts said the U.S. Defense Department and Central Command have been warning of the increasing activity of a Sunni insurgency network in northern Syria. They said the Pentagon has been discussing a U.S. strike that could end the network's operation.

"The Syrians appear to be encouraging some of this activity," a diplomat said. "It's clear that diplomacy hasn't worked and Washington is considering other ideas."

The insurgency network was said to be based in Aleppo, the sources said. They said fighters from such countries as Algeria, Egypt, Jordan, Kuwait, Morocco, Lebanon, Sudan and Tunisia arrive in Aleppo and undergo recruitment and training by Sunni insurgency groups linked to Abu Mussib Al Zarqawi, head of Al Qaida in Iraq.