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, according to Reuters.

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, Reuters says.

While U.S. officials have not made a formal request to the Jordanian government, the country was widely considered a top choice to host the training due its close security relationship with Washington, proximity to neighboring Syria and pool of more than 600,000 Syrian refugees.

U.S. officials and analysts said Jordan fears violent retaliation from Syria if its territory is used for overt training conducted by U.S. military units.

"Jordan told the U.S., 'No boots on the ground'," said one of the officials, who all requested anonymity because they were discussing sensitive U.S. military arrangements.

Other current and former U.S. officials described the Jordanian position as less ironclad, however, and said they still hoped to convince Jordan to participate in the program, which must still be approved by the U.S. Congress.

A Jordanian official in Amman, speaking on condition of anonymity, told Reuters it was "premature to even suggest the Kingdom has rejected any such plan that even the Americans have yet to flesh out."

While there are other potential sites where the training could take place, including Turkey and Gulf states such as Saudi Arabia, no agreements have been struck, U.S. officials said. Turkey and the Saudis also have sensitivities about the presence of large numbers of U.S. troops.

"There’s been no decision on location, at all. Or even ... what the character of the program itself would look like, if we get the money" from Congress, said a second U.S. official.

Jordan's King Abdullah met with U.S. Vice President Joe Biden in Washington this week for talks that included Syria, the White House said in a statement. U.S. and Jordanian officials declined to give further details.

If approved by lawmakers, the $500 million fund to arm and train rebels will not be available until Oct 1 at the earliest, or possibly months later depending on potential delays in the U.S. Senate and House of Representatives.

Other important details, including how "moderate" rebels would be vetted to weed out those with records of human rights abuses or ties to extremist groups, have yet to be finalized, the U.S. officials said.