Jordan, where the U.S. Central Intelligence Agency has been covertly training Syrian rebels for more than a year, is reluctant to host an expanded rebel instruction program, U.S. officials said.

Jordan’s reticence, confirmed by four U.S. officials, is a potentially serious setback for President Barack Obama’s proposed $500 million initiative, announced in June, to train and arm moderate rebels fighting the government of Syrian President Bashar al-Assad and al Qaeda-linked groups.

It could signal a larger challenge in finding suitable nations willing to host the U.S.-led training at a time of heightened tensions across much of the Middle East. […]