* Jordan plays ambivalent role in Syrian conflict
* Wants Assad to quit, but wary of Islamist militants
* Fears of al Qaeda crimp foreign support for rebels
The growing power of Islamist fighters in southern Syria is causing alarm in neighbouring Jordan, which backs rebels battling President Bashar al-Assad but fears those linked to al Qaeda.
Similar concerns among Syria's other neighbours, including Turkey and Israel, are complicating an already disjointed world response to the bloody turmoil at the heart of the Middle East.
Jordan has allowed limited U.S. military training of rebels on its territory. Some other fighters have crossed from the kingdom into Syria, although others, especially Islamists, have been intercepted and even put on trial.
Eighteen months ago, Jordan's King Abdullah was the first Arab leader to urge Assad to step aside, but he used a visit to Washington last week to voice Jordan's concern over "militant terrorist organisations" gaining ground along Syria's southern frontier with the kingdom.
His comments in the Oval Office alongside U.S. President Barack Obama underline fears that Jordan's national security is now threatened by Islamists in Syria whose hatred of Assad is matched only by their hostility to the pro-Western monarchy.
As a result, senior diplomats in Amman say, Jordan has resisted pressure from Gulf Arab states to step up arms shipments to rebels it believes might one day turn against it.
Jordan is also concerned that Syria, which is widely believed to possess chemical weapons, might lash out in reprisal for any heightened Jordanian support for insurgents.
"The fire will not stop at our border and everybody knows that Jordan is as exposed as Syria," Assad said two weeks ago in an interview which depicted al Qaeda as a security concern for both countries - a message which resonated with many Jordanians.
Syria's rebel Nusra Front, one of the deadliest forces fighting to topple Assad, declared its allegiance to al Qaeda leader Ayman Zawahri earlier this month, formally cementing an alliance with a group which has targeted Jordan in the past.
At the same time, Nusra Front fighters and other rebels have opened a new battlefront in southern Syria, a move which Assad blamed on the infiltration of thousands of militants from Jordan, seizing military posts and swathes of land.
"The security threat comes from the Nusra Front and the radical Islamic groups - if they win and are stationed on the Jordanian border, that causes problems from the army's perspective," said retired Jordanian Major-General Fayez Dwairi.
Jordanian officials, who asked not to be identified, said limited security cooperation with Damascus was continuing.
As well as the danger of fighting spilling over its border from Syria's southern province of Deraa, Amman faces the familiar threat of young Jordanians joining a regional conflict and then returning home, battle-hardened and radicalised.
Jordanian-born Abu Musab al-Zarqawi travelled to Afghanistan in the 1980s and also fought in Iraq, from where he was believed to have planned attacks on hotels in Amman which killed dozens of people in November 2005.
With the Syrian border just 75 miles (120 km) from Amman, the conflict in Jordan's northern neigbour is much closer to Abdullah's capital than the turmoil in Iraq ever was.
So although Jordan has allowed the training of rebels on its territory and permitted some Gulf-funded arms shipments into Syria, it has rebuffed pressure to send larger consignments into the war zone, according to diplomats.
"Jordan national strategic interests come first and before any Gulf agenda," said a senior security official who asked not to be named, singling out Qatar for what he said was a flawed policy of empowering the Muslim Brotherhood, a longtime adversary of Jordan's monarchy.
The Brotherhood's political wing in Jordan, the Islamic Action Front, has taken part in protests calling for political reform and denouncing subsidy cuts in the resource-poor kingdom, which relies on Gulf grants to narrow its gaping budget deficit.
Publicly, Jordan says it remains neutral in the conflict.
Prime Minister Abdullah Ensour has even denied in parliament that Jordan hosts rebel training programmes - contradicting accounts from rebels and diplomats - and said the kingdom opposed any military intervention to overthrow Assad.
Jordan's jitters were illustrated last month, rebels say, when it warned them not to seize the main Nasib border crossing. They said Jordanian forces also detained and interrogated local weapons smugglers to curb the flow of small arms into Deraa.
Shortly afterwards Washington announced it would send an Army headquarters unit - which could theoretically command combat troops. Jordan has also beefed up its military capability on the border and requested U.S. Patriot batteries to protect it from any retaliatory missile attack from Assad's forces.
Syrian rebels have received military supplies via Turkey, which also frets about the rise of radical Islamists among the insurgents, fearing greater security problems along its 900 km border should they rise to prominence in a post-Assad Syria.
Those concerns are limiting direct support to the opposition from Syria's northern neighbour, Turkish officials say.
"Turkey is at least as concerned as the U.S. and other allies about al Nusra. Turkey also thinks that the continuation of the current situation will feed more extremism," said a source close to the Turkish government.
Disunity among rebels, rival allegiances and Western fears of being sucked into an open-ended conflict in which Russia and Iran back Assad have already obstructed any concerted foreign response to the crisis.
But doing nothing will let the balance of forces on the ground shift towards radical Islamists, said Sinan Ulgen of the Istanbul-based Centre for Economic and Foreign Policy Studies.
Another neighbour watching anxiously is Israel, whose forces occupy the Golan Heights southwest of Damascus and which shares Jordan's concerns over the spread of Islamist fighters.
Western diplomats familiar with Jordanian thinking and rebel commanders on the ground say Jordan appears to be channelling weapons to moderate rebels inside Syria to protect its border - a move which might also indirectly address Israeli concerns.
The two countries signed a peace deal in 1995 and have worked closely together since then on regional security.
But Amman's limited support frustrates rebels who say that rudimentary two-week weapons training for hundreds of fighters will do little to tilt the military balance against Assad.
"There are growing suspicions about Jordan's ultimate goals in (Syria) and the way they are starting to form militias loyal to them," said Abu Haytham, a rebel commander of Ababeel Hauran, an Islamic brigade. He accused the kingdom of backing Syrian forces ready to confront the Islamists after Assad falls.
Another rebel commander, Abu Omar, who lost an eye in street fighting in the southern city of Deraa, cradle of the revolt against Assad, called for more sophisticated weaponry.
"If the Jordanians supply us with anti-aircraft and anti-tanks weapons we will finish Deraa in a week, and not much later we will be at the gates of Damascus," the 37-year-old former trader told Reuters in Jordan's northern city of Irbid where he was being treated for shrapnel wounds.
Video footage of rebels in southern Syria has shown them wielding anti-tank weapons and heavy calibre rifles. Some appeared to have been supplied from outside Syria, but rebels say most were seized from captured army bases.
According to regional diplomats, whose accounts tally with those of rebels spoken to by Reuters, Jordan's intelligence services sent a few shipments of light arms to a Jordanian-backed rebel military council, but the supplies soon dwindled.
As one rebel commander described it: "The Jordanians would say, 'Wait a bit, a weapons shipment is coming'. The rebels would wait one day, two days, or a month... In the end, the weapons that arrived were damaged and insufficient."