Britain should avoid becoming embroiled in direct military action in Syria, according to the former intelligence director of MI6.

On Thursday, the US Senate approved President Barack Obama's plan to train and arm moderate Syrian rebels to fight Islamic State (Isis), endorsing a key plank of the president's plans for taking on jihadi militants.

US air strikes are also expected in Syria as part of Obama's strategy for wider military action in the country against Isis.

Nigel Inkster, a former deputy head of MI6, said that, although empowering rebel forces was sensible, the UK should not be tempted to join any potential military action in Syria which would antagonise the allies of President Bashar al-Assad, such as Russia. Britain has not ruled out air strikes in Iraq or Syria, but it has said targeting Isis positions in Syria would be complicated.

Inkster said: "Military activity that takes place in Iraq will take place with the consent of the Iraqi government. In the case of Syria, that is not the case … any such activity would technically be an act of war.

"You can be confident that Assad's allies would be very quick to make this point. But from a military perspective the logic of such an engagement is inevitable because ultimately Syria is where this force needs to be defeated. The emphasis has to be on local actors, enabling local Syrian actors. They had some success previously [against Isis] but then they had logistical problems, running out of equipment just at the point Isis was acquiring new supplies."

His comments followed confirmation that France conducted its first air strike on Friday against Isis targets in Iraq, joining the US as part of an international coalition against the militants.

Britain has yet to decide whether to join the air strikes, although Inkster said that one overlooked aspect of UK involvement would be putting individuals inside Iraq, to obtain what military sources call "ground truth".

Meanwhile, it emerged that almost 50 hostages, including children, who were seized by Isis militants in the Iraqi city of Mosul in June, are safely back in Turkey.

The fate of British hostage Alan Henning remains uncertain despite an appeal by British Muslim scholars to Isis to free the 47-year-old aid worker whom they are threatening to behead.

Briton John Cantlie is also being held by Isis. Photojournalist Cantlie, 43, appeared on an Isis video last week in which he urged the UK and US governments to negotiate with his captors. The admission by US army chief of staff Ray Odierno on Friday that it might be necessary to deploy more US troops than the 1,600 already in Iraq are still being digested. Inkster said Iraqi troops were sufficient. "In terms of numbers, the Iraqi army, the peshmerga, have probably got enough to do the job, but they will have to be rebuilt and reinvigorated. Whether that can be done is the moot point."

Inkster, who now works for the International Institute of Strategic Studies as director of transnational threats and political risk, said Islamic State was eminently "beatable" in military terms.

"The international perception – their own perception – is that they are on a roll, but they are not an unstoppable force and within their own hinterland they do have vulnerabilities. The more they try to expand, the more they risk overextending."

Inkster worked for MI6 from 1975 until 2006 and spent seven years on the board of the intelligence service, the last two as assistant chief and director for operations and intelligence.