WASHINGTON—The Pentagon is stepping up plans to deal with a dangerous regional spillover from Syria's possible collapse—a scenario it had recently seen as remote—drawing up proposals including a Jordanian buffer zone for refugees secured by Arab troops, said U.S. officials familiar with the discussion.

The plans seek to minimize direct U.S. involvement, but they reflect a reassessment of the Pentagon's hands-off approach. The shift comes after Defense Secretary Chuck Hagel's trip last month to the Middle East, during which Arab leaders appealed for the U.S. to focus on the danger of Syria's disintegration into warring sectarian fiefdoms.

"The Syria message was loud and strong," said a senior diplomat briefed on Mr. Hagel's trip. "Everybody's scared. And nobody knows what the hell we are going to do there."

The U.S. fear is that Syria could break apart and fighting and additional refugees could spill into Jordan—an American ally and peace partner with Israel—threatening it and other U.S. interests in the region, these people say.

A buffer zone along the Syria-Jordan border, if put into effect, would provide a way to shelter a Syrian refugee population that is overwhelming Jordan, the U.S. officials said. It would also be a possible conduit for the U.S. and Europeans to funnel aid, including arms, to Syrian rebels, they said.