Western training of Syrian rebels is under way in Jordan in an effort to strengthen secular elements in the opposition as a bulwark against Islamic extremism, and to begin building security forces to maintain order in the event of Bashar al-Assad's fall.

Jordanian security sources say the training effort is led by the US, but involves British and French instructors.

The UK Ministry of Defence denied any British soldiers were providing direct military training to the rebels, though a small number of personnel, including special forces teams, have been in the country training the Jordanian military.

But the Guardian has been told that UK intelligence teams are giving the rebels logistical and other advice in some form.

British officials have made it clear that they believe new EU rules have now given the UK the green light to start providing military training for rebel fighters with the aim of containing the spread of chaos and extremism in areas outside the Syrian regime's control.

According to European and Jordanian sources the western training in Jordan has been going on since last year and is focused on senior Syrian army officers who defected.

"As is normal, before any major decision is taken on this issue, the preparations are made so that when that decision is taken, everything is in place for it to go smoothly. That is what these groups [special forces] do. They go in in advance," a European diplomat said.

A Jordanian source familiar with the training operations said: "It's the Americans, Brits and French with some of the Syrian generals who defected. But we're not talking about a huge operation."

He added that there had so far been no "green light" for the rebel forces being trained to be sent into Syria. But they would be deployed if there were signs of a complete collapse of public services in the southern Syrian city of Daraa, which could trigger a million more Syrians seeking refuge in Jordan, which is reeling under the strain of accommodating the 320,000 who have already sought shelter there.

The aim of sending western-trained rebels over the border would be to create a safe area for refugees on the Syrian side of the border, to prevent chaos and to provide a counterweight to al-Qaida-linked extremists who have become a powerful force in the north.

British officials say new European guidelines on the Syrian arms embargo, formally adopted by the EU at the beginning of March, allow military training as long as the ultimate aim of that training is "the protection of civilians".

Paris takes an identical view of the EU rules.

Officials in Brussels say the language of the guidelines is less than clear-cut. "It's deliberately hazy," said one. "When it comes to technical assistance, what it means in practice depends on who you ask. The Brits and the French, for example, are much more forward-leaning than others. The principle is that the assistance should be for the protection of civilians, but as we saw in Libya, that can be interpreted in different ways."

British officials argue that training of Syrian forces to fill the security vacuum as the Assad regime collapses would be help safeguard civilian lives.

William Hague, the foreign minister, outlined the goals of such training on Wednesday.

"Such technical assistance can include assistance, advice and training on how to maintain security in areas no longer controlled by the regime, on co-ordination between civilian and military councils, on how to protect civilians and minimise the risks to them, and how to maintain security during a transition," he told parliament. "We will now provide such assistance, advice and training."

A Foreign Office spokesman said: "It's not the sort of thing we are going into too much detail on right now. We are big on the transition picture, because at some point Assad is going to fall, and the opposition are going to need help to provide governance in areas they control, and that of course includes security. But security doesn't just mean fighting, it also means basic law and order, and policing."

The Pentagon said last October that a small group of US special forces and military planners had been to Jordan during the summer to help the country prepare for the possibility of Syrian use of chemical weapons and train selected rebel fighters.

That planning cell, which was housed at the King Abdullah II Special Operations Training Centre in the north of the capital, Amman, has since been expanded to co-ordinate a more ambitious training programme. But Jordanian sources said the actual training was being carried out at more remote sites, with recent US reports saying it was being led by the CIA.

For the first two years of the Syrian civil war, Jordan has sought to stay out of the fray, fearing a backlash from Damascus and an influx of extremists that would destabilise the precariously balanced kingdom.

"What has happened of late is that there has been a tactical shift," said Julien Barnes-Dacey, a Middle East expert at the European Council on Foreign Relations thinktank. "Islamist forces have been gaining steam in the north and Jordan is keen to avoid that in the south. Having been very hands-off, they now see that they have to do something in the south."

He added: "There is a feeling that Jordan simply can't handle a huge new influx of refugees so the idea would be to create a safe zone inside Syria. For them it's a no-win scenario. Everything they had been seeking to avoid has come to pass."

For western and Saudi backers of the opposition, Jordan has become a preferable option through which to channel aid than Turkey. Ankara has been criticised for allowing extremist groups, such as the al-Nusra Front, become dominant on the northern front while it focused on what it sees as the growing threat of Kurdish secessionism.

"The Americans now trust us more than the Turks, because with the Turks everything is about gaining leverage for action against the Kurds," said a Jordanian source familiar with official thinking in Amman.

The US has announced an extra $60m (£40.2m) in direct aid to the rebels, including military rations and medical kits. Asked on Tuesday whether assistance included military training, the US state department spokesman Pat Ventrell replied: "I really don't have anything for you on that. Our policy has been non-lethal assistance."

Earlier this week, the US secretary of state, John Kerry, said Washington was now confident that arms supplies to the rebels would not be diverted to extremists. "There is a very clear ability now in the Syrian opposition to make certain that what goes to the moderate, legitimate opposition is, in fact, getting to them, and the indication is that they are increasing their pressure as a result of that," he said.

Syrian rebels have said that in the past few months there had been a relaxation of the previously strict US rules on what kinds of weapons were allowed across the border, and that portable anti-aircraft missiles had been released from Turkish warehouses where they had been impounded.

Matt Schroeder, who tracks the spread of such weapons for the Federation of American Scientists, said the recent appearance of modern, sophisticated anti-aircraft missiles in the hands of such fragmented rebel groups was deeply troubling in view of their capacity to bring down civilian airlines.

"This is a step above anything we've seen before in the hands of non-state actors," he said. "This is a new and unfortunate chapter in recent manpad [man-portable air-defence] proliferation."