WarSclerotic

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.

Associated Press

Israeli soldiers held exercises in the Golan Heights near Syria on Tuesday.

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.

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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.

In recent days, some of the most pessimistic U.S. scenarios for the war’s trajectory appear to have been confirmed—with claims of chemical-weapons use in Syria, back-to-back airstrikes inside the country that have been attributed to Israel and reports early this week that several thousand Iran-backed Hezbollah militants are fighting alongside Syrian regime forces. On Tuesday, a Syrian rebel group abducted four United Nations peacekeepers in the Golan Heights, the U.N. said.

The prospect of a Syrian breakup was raised Tuesday by Secretary of State John Kerry, who met in Moscow with his Russian counterpart, Sergei Lavrov, in an effort to move Syria’s warring sides toward a negotiated solution.

“The alternative is that there is even more violence. The alternative is that Syria heads closer to an abyss, if not over the abyss and into chaos,” Mr. Kerry told reporters. “The alternative is that there may be even a breakup of Syria.”

As part of the military’s stepped-up contingency planning, a U.S. Army command team being deployed this month will work with Jordan’s military on options that include planning for a buffer zone, the U.S. officials said. The Pentagon announced last month that it was sending Army troops to bolster its presence in Jordan but didn’t specify that role.

U.S. and Jordanian officials have been privately discussing the buffer zone idea for months, a senior administration official said. Jordan’s embassy in Washington had no immediate comment.

U.S. military leaders have also suggested increasing the number of trainers working with Jordanian forces, according to officials. The military could offer air support, including embedding U.S. Air Force tactical controllers in Jordanian ground units, these officials said.

Egyptian officials have told their American counterparts that Cairo would be prepared to be part of an international stabilization force in Syria “under certain circumstances,” according to a senior American official. Egyptian officials in Washington had no immediate comment.

For months, the White House has debated options that would boost the U.S. role in supporting more moderate factions battling Syria’s President Bashar al-Assad. Leaders in the region privately complained the U.S. wasn’t being decisive enough and urged the U.S. to set a policy path that would corral Arab states that have often worked at cross-purposes, U.S. officials said—asking the U.S. to play a “midwife” role, as one of these people put it.

In recent weeks, Arab leaders have traveled to Washington to put their cases directly to President Barack Obama. Jordan’s King Abdullah privately told top White House policy makers last month that Syria could become a new al Qaeda safe haven, according to senior U.S. officials.

In the Mideast, Mr. Hagel listened as Egyptian Defense Minister Abdel-Fattah el-Sissi told him Syria’s breakup would be a “huge disaster” for all sides—not only for King Abdullah but for U.S. interests in the region, according to U.S. officials.

Mr. Hagel came away from the trip, which also included stops in Israel, Jordan, Saudi Arabia and the United Arab Emirates, “seeing storm clouds on the horizon,” one of Mr. Hagel’s advisers said. Last week, he became the first U.S. official to say publicly that the administration was weighing arming the rebels.

The contingency planning shows the extent to which Pentagon leaders now see a breakup as a real threat to U.S. interests in the region. Providing support to rebels risks inadvertently arming forces allied to al Qaeda. Standing back could lead to a disorderly collapse, including the possibility that Jihadist militias emerge with territory to govern. The potential for a disruptive spillover has spurred renewed interest in ideas the Pentagon and White House had previously ruled out.

The White House remains cautious about commitments that could draw the U.S. into the war and has been treading gingerly given the constrained options.

On Tuesday, President Obama said the U.S. has moral and national-security obligations to respond to bloodshed in Syria but continued to urge caution about what he said was “perceived” use of chemical weapons in the Middle Eastern country.

Officials said it could be easier to win support for expanded American military involvement for efforts aimed at humanitarian relief. The growing refugee problem could also provide legal justification for intervening in Syria if the U.S. determines that key allies such as Jordan are seriously threatened, U.S. officials believe.

But how to create a buffer is controversial. Syria has repeatedly said that establishing such zones in the north or south of the country would be considered an act of “foreign aggression.” A safe zone inside Jordan, meanwhile, could also end up drawing more refugees to the beleaguered kingdom.

Americans for now envision any buffer zone being set up on Jordanian soil, where U.S. troops could be stationed. Some Jordanian officials want it to be on Syrian territory, U.S. officials say, which would limit involvement of American personnel. The U.S. has so far ruled out putting troops on the ground inside Syria but could send intelligence officers.

Cash-strapped Jordan has taken in 520,000 refugees from Syria, straining an already anemic economy and sowing public discontent. King Abdullah has referred to one camp, which houses 110,000 to 120,000 Syrian refugees, as Jordan’s fifth-largest city. Protests against poor living conditions at the camp are common. A growing number of Syrians want to return home.

The U.S. faces a difficult balancing act in shoring up King Abdullah without undercutting him. A large overt U.S. military presence in Jordan could backfire and fuel unrest aimed at the king. Islamists already see the kingdom as an arm of the U.S. government and Israel, and there have been several protests demonstrating against U.S. intervention in Syria.

Jordan is concerned about the Jihadi threat, especially the Al Nusra Front, an arm of al Qaeda that plays a central role in the fight against Mr. Assad and which has recruited hundreds of Jordanian nationals. A senior European defense official said Jihadists were eager to “settle scores” with the kingdom for providing aid to the U.S. against al Qaeda.

The U.S. military’s Joint Chiefs of Staff started presenting military options to the White House in July 2012 that ranged from training and arming moderate rebel factions to conducting airstrikes on Syrian air defense and leadership targets.

Of those, Mr. Obama has authorized nonlethal support from the State Department, a limited Central Intelligence Agency training program and deployments of U.S. military personnel to train Jordanian forces in how to deal with Syria’s chemical-weapons threat.

More recently, Mr. Obama has moved toward authorizing the U.S. to provide body armor and night-vision goggles to the rebels, and has revived the option of arming some rebel factions—a proposal he rebuffed last year.

Officials said the buffer zone along the Jordan-Syria border could be the transit point for arms and aid distribution, possibly to include gear to protect fighters from chemical and biological weapons.

“Syria affects so much of the Middle East, nothing happens independently in the Middle East,” Gen. Ray Odierno, the Army chief of staff said on Tuesday. “If we don’t get this right, it could change the whole face of the Middle East.”

—Paul Sonne in Moscow, Joe Lauria at the U.N. and Suha Maayeh in Amman, Jordan, contributed to this article.Write to Adam Entous at adam.entous@wsj.com and Julian E. Barnes at julian.barnes@wsj.com

A version of this article appeared May 8, 2013, on page A1 in the U.S. edition of The Wall Street Journal, with the headline: Pentagon Plans for the Worst in Syria.