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

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 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 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 neighbouring Syria and a pool of more than 600,000 Syrian refugees.

US officials and analysts said Jordan fears violent 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 programme, which must still be approved by the US Congress.

The Jordanian government, through its Washington embassy, declined requests by Reuters news agency for comment.

A Jordanian official in Amman, 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 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 Assad's opponents.

But it faces increasing threats to its stability from the Syrian conflict and rise of self-declared jihadist groups such as the Islamic State. The US has already increased its military presence in Jordan to around 1,300 soldiers.

It has also stationed Patriot surface-to-air missiles there.

'Moderate' rebels

Jordan's King Abdullah met US Vice President Joe Biden in Washington DC on Thursday for talks that included Syria, the White House said in a statement.

The US and Jordanian officials declined to give further details.

If approved by legislators, the $500m fund to arm and train rebels will not be available until October 1 at the earliest,or possibly months later depending on potential delays in the US 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 finalised, the US officials said.

US law requires the State Department to screen foreign military members and units being trained with US funds.

Assistance is barred when credible evidence of human rights abuses turns up. But it is unclear how the law applies to a proxy force like the Syrian rebels.