By Suleiman Al-Khalidi

AMMAN (Reuters) - Bashar al-Assad's first public warning to Jordan over its role in channeling Islamist Sunni Muslim rebels to southern Syria, close to Damascus, points to a president increasingly rattled by the threat of a push against his stronghold in the capital.

Assad told Jordan this week it would be playing with fire by supporting the rebels, saying the Western-backed kingdom was just as vulnerable as his country to al Qaeda militants gaining ground in Syria's two-year conflict.

His comments came after weeks of fighting in southern Syria, where rebels have seized military bases, made advances close to the Jordanian border and the Israeli-occupied Golan Heights, and cut two main roads to Damascus.

Assad said he had sent envoys to Jordan in the last two months to challenge authorities over reports of Syrian rebels being trained in Jordan and thousands of fighters being allowed to infiltrate from Jordanian territory into Syria.

"He is trying to communicate this message: 'Don't do this guys'. He realizes that we are threatened by the same radical forces that are a threat to them as well," said one Jordanian intelligence expert who requested anonymity.

Ideologically there is little common ground between Assad's Syria, an Iranian-backed republic which backs anti-Israel militants, and King Abullah's Jordan, a U.S.-allied monarchy which has made peace with the Jewish state.

But in an interview broadcast on Wednesday the Syrian leader reminded Amman of the two countries' shared goal of fighting al Qaeda, a joint concern which appears to have prevented Assad so far from grouping Jordan together with his arch-foes Turkey and Gulf Arab states which openly support arming the insurgents.

Assad said some officials in Jordan did not appreciate the level of danger which Syria's crisis placed their own country in. "The fire does not stop at our border and everyone knows that Jordan is exposed to what Syria is exposed to," he said.

Last week the Nusra Front, which has claimed responsibility for scores of car bombings and offensives against Assad's troops, formally declared allegiance to al Qaeda, whose followers have carried out attacks in Amman in recent years.

JORDAN VULNERABLE

"It's an implied indirect threat...that Jordan is vulnerable. (Assad) is telling Amman to watch out," said a senior Western diplomat in Amman.

The aid-dependent kingdom is engaged in a perilous balancing act in response to the Syrian war, which is attracting Sunni Muslim militants who view the pro-U.S. monarchy with just as much hostility as they do the Alawite Syrian leader.

Jordanian and defense and intelligence experts say security cooperation with Syria has continued throughout the conflict and say this was underlined by Assad's disclosure of a visit to Amman by Syrian intelligence chief Ali Mamlouk.

"We sent a security official about a month ago...and briefed them on the assessments we have and he heard a complete denial of Jordanian involvement in all that is happening," Assad said of Mamlouk's visit.

Jordanian officials said the mission signaled that Damascus wanted to deal privately with any differences, but Assad made clear he was unconvinced by the kingdom's denials.

"There was the escalation in Deraa and we saw thousands of gunmen and terrorists coming from Jordan with their weapons and ammunition," he said, although he added that Syria had no proof of its own about rebel training camps inside Jordan.

Assad also played on Jordan's political establishment concerns over the ascendancy of the Muslim Brotherhood, the country's biggest political opposition party that has led street protests calling for political reforms and constitutional changes to whittle away King Abdullah's powers.

"We wish they would learn from the lessons we learnt from the Muslim Brotherhood era," he said referring to his Alawite minority's bloody confrontation with the Syrian Muslim Brotherhood that cost thousands of lives in the 1980s.

Assad's message resonates with many in Jordan's political elite, who caution against any action that would trigger retaliation of a powerful neighbor with a record of sending agents to destabilize the kingdom during past tensions.

"It's not in the interests of Jordan to open military training sites. It's inviting trouble. If one side or the other wins, we could lose. And what for?" said Mamoun Abu Nowar, a retired Jordanian airforce general.

"Assad may have been cornered but he still has a lot of power to do untold damage," he added.

Defense strategists say Assad may be reluctant to get tougher with Jordan for now because of the strategic importance of the southern border, which offers a direct route north towards his seat of power in Damascus.

"If Jordan opens its front, the West will get to Bashar in an hour after a campaign of heavy aerial bombardment and they would be spared a guerilla war. This is why Assad is not breaking with Jordan," Abu Nowar added.

(Editing by Dominic Evans and Mike Collett-White)