Jordan's reticence, confirmed by four US officials, is a potentially serious setback for President Barack Obama's proposed $500m initiative, announced in June, to train and arm Syria militants.

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

While US 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 the US, proximity to neighboring Syria and a pool of more than 600,000 Syrian refugees.

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

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

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

A Jordanian official, speaking on condition of anonymity, said 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 Persian Gulf states such as Saudi Arabia, no agreements have been struck, US officials said. Turkey and the Saudis also have sensitivities about the presence of large numbers of US troops.

Jordan already hosts a small and ostensibly covert effort by the CIA to equip and train small groups of anti-Syria militants.

Syria has been gripped by a foreign-sponsored insurgency war since March 2011. An estimated 160,000 people have reportedly been killed and millions of others displaced due to the crisis fueled by the Western-backed militants.

NTJ/HH