Syria’s regime sternly warned neighboring Jordan on Thursday that it was “playing with fire” by allowing the U.S. and other countries to train and arm rebels on its territory.

Jordan, America’s closest ally in the Arab world, has long been nervous that President Bashar Assad’s hard-line regime could retaliate for supporting the rebels. The warning carried on state media may add to those jitters, though Jordanian government officials publicly downplayed it as “mere speculation by the Syrian media.”

Syrian state television said leaks in U.S. media show Jordan “has a hand in training terrorists and then facilitating their entry into Syria.” State radio accused Jordan of “playing with fire.”

A front-page editorial in the government daily al-Thawra accused Amman of adopting a policy of “ambiguity” by training the rebels while at the same time publicly insisting on a “political solution” to the Syrian crisis.

“Jordan’s attempt to put out the flame from the leaked information will not help as it continues with its mysterious policy, which brings it closer to the volcanic crater,” the paper said.

Two Jordanian officials downplayed the diplomatic tiff with Syria. One said Jordan will not discuss the state of relations through the media.

“Such discussions are usually carried out through the appropriate diplomatic channels,’ he said. Both officials insisted on anonymity out of concern that their comments may further irritate relations, which have been historically bumpy.

Jordan has long been concerned that the Assad regime could use chemical weapons against it, or that agents linked to the regime or its allied Lebanese militant group Hezbollah could attack the kingdom.

The Syrian warnings appeared to reflect the regime’s concerns about statements by U.S. and other Western and Arab officials saying Jordan has been facilitating arms shipments and hosting training camps for Syrian rebels since last October.

The training and the influx of foreign-funded weapons have coincided with rebel gains in southern Syria near the strategic border area with Jordan.

Those gains could be leading up to control of the region along the Jordanian border. That would be a major victory that could offer rebels a staging ground to try to attack the capital Damascus, the seat of Assad’s power.

Rebels already control large swathes of territory in northern Syria along the Turkish border.

Activists reported more advances in the south on Thursday.

The rebels receiving training in Jordan are mainly secular Sunni Muslim tribesmen from central and southern Syria who once served in the army and police.

The force is expected to fill a security vacuum — mainly to protect the border with Jordan, assist displaced Syrians and possibly set up a safe haven for refugees — if Assad is toppled.

The Syrians training in Jordan are also envisioned as a counterbalance to the Islamic militant groups that have proven to be among the most effective of the myriad rebel factions fighting Assad’s forces on the ground.

Chief among those rebel extremist groups is Jabhat al-Nusra or the Nusra Front, which the U.S. designates as a terrorist group and says is associated with al-Qaida.

The prominence of such extremist groups has fueled fears in Jordan that the chaos in Syria could lead to a failed state where Islamic militants have a free hand.

Israel and the United States also are concerned about militants potentially operating in the area near the Israeli frontier with Syria in the Golan Heights — also in the south — should Assad’s regime collapse.