The United States has sent troops to Jordan to bolster its military capabilities in the event Syria's civil war escalates, U.S. Defense Secretary Leon Panetta said on Wednesday.

Speaking at a NATO conference of defense ministers, Panetta said the United States has been working with Jordan to monitor chemical and biological weapons sites in Syria and also to help Jordan deal with refugees pouring over the border from Syria.

About 150 U.S. troops, largely Army special operations forces, are working out of a military center near Amman, two senior defense officials said on condition of anonymity. The troops have moved back and forth to the Syrian border as part of their work, which is joint planning and intelligence gathering, one official said.

The revelation of U.S. military personnel so close to the 19-month-old Syrian conflict suggests an escalation in the U.S. involvement in the conflict, even as the Obama administration pushes back on any suggestion of a direct intervention in Syria.

News of the U.S. mission to Jordan also follows several days of shelling between Turkey and Syria, an indication that the civil war could become a regional conflict. ASSOCIATED PRESS