LONDON: Jordan is facing mounting tension with neighbouring Syria amid signs that it has moved to a more active role in support of the rebels fighting to overthrow President Bashar al-Assad’s government.

The border between the countries was reinforced on the Jordanian side on Thursday, after Syrian state media warned the western-backed kingdom it was “playing with fire” and poised “on the edge of a volcano” by backing the opposition.

Recent weeks have seen a spate of reports about arms shipments from Jordan to anti-Assad rebels who have been making gains around Deraa, the Syrian city closest to the border. Opposition sources say the military situation reflects enhanced supplies and training.

Barack Obama discussed the crisis with King Abdullah II in Amman on his Middle East tour last month. Jordan was the only Arab state the president visited — an indication of the pressure the king is under to be more supportive of the effort to weaken Assad and drive him from power.

Diplomats have described the discussion of plans for a buffer zone in southern Syria as well as accelerated training for rebel fighters by the US and Jordan. British and French special forces are also reported to be involved in training.

In an apparent reflection of nervousness about the issue, a government spokesman in Amman insisted on Friday that Jordan was “not part of the conflict” in Syria and maintained its support for a “peaceful solution” — the formal stance of all Arab states. But the spokesman refused to comment either on the training or the buffer zone, the Al-Ghad newspaper reported.

The Washington Post cited Jordanian security officials this week as saying that a plan to complete the training of 3,000 Free Syrian Army officers by the end of June has been brought forward to the end of April in light of the border victories. The FSA is backed by western and Arab governments as a bulwark against the rise of radical Islamist groups.

Jordanian sources describe a “double discourse” — an official one that reiterates the formal position alongside clandestine training and Saudi-financed arms supplies delivered with the help of the CIA. Jordan’s powerful Mukhabarat secret service enjoys a close relationship with its western partners, including MI6 (Britain’s foreign intelligence service).

“The Jordanians are happy to channel support but they say ‘don’t put us in the front line’,” said a Syrian opposition figure. “They used to be afraid that Assad’s intelligence system could hit back and hurt Jordan but now he is weak they feel emboldened to be more active.”

Jordanian officials repeatedly speak of the gravity of the Syrian crisis, with concern focusing on the flow of refugees across the border and the risk that extremist elements will come with them.

“Jordan can’t sit idle and watch Al Qaeda and other militants seizing control of its... border with Syria,” Jordan’s information minister, Sameeh Maaytah, was quoted as saying. “It must take proactive steps to arrive at a state of equilibrium in the security structure on the border.”

An estimated 460,000 Syrian refugees are in Jordan. In one 24-hour period this week 1,967 arrivals were recorded. If the influx continues at the current rate Jordan could be hosting more than one million refugees by the end of 2013. Abdullah Ensour, the newly appointed prime minister, has warned publicly of a “catastrophic” situation” and used even stronger language in private, according to sources in Amman.

Domestic strains have also been evident in complaints about the number of Syrian refugees and several MPs urging the closure of the border. Abdul Karim al-Dughmi, a conservative politician, criticised the government’s “timid position” on the crisis and blamed a “conspiracy by some Arab states” for the unrest.

By arrangement with the Guardian