Hundreds more British troops are being sent to Iraq next month to bolster the fight against Islamic State (Isis) militants, compounding fears of “mission creep.”

The defence secretary, Michael Fallon announced an additional deployment of British combat-ready troops numbering “in the low hundreds” to help train local forces battling Isis forces, who control vast swathes of northen Syria and neighbouring Iraq.

Fallon said details of the contribution to an international mission were still being finalised but would likely include a small contingent of combat-ready British soldiers at four US-led “safe” centres, one in Kurdistan and three near the Iraqi capital Baghdad.

The move represents a significant increase of the 50-strong British force currently helping Iraqi and Kurdish fighters prepare to retake territory seized by the jihadist movement over the past year.

Last month General Martin Dempsey, the most senior officer in the US military, revealed that the US is actively considering the direct use of troops against Isis in Iraq, days after Barack Obama doubled troop levels there.

For months the US airforce and RAF planes have been heavily involved in air strikes and reconnaissance missions across Iraq that are credited with helping slow the advance of Isis fighters and have now paved the way for ground operations. Fallon revealed that the RAF had flown a “huge number” of missions above Iraq second to the United States and five times as many as France.

He also suggested that a key element of the UK contribution would be passing on the experience gained during previous operations in Iraq – the last British troops to withdraw from the country left as recently as May 2011 – and also lessons learned during the UK military’s 13-year war with the Taliban in Afghanistan where troops dealt routinely with roadside bombs and other explosive devices.

Yet the defence secretary was most keen to stress that the emphasis on increased British involvement was on ensuring local forces are sufficiently trained and equipped to push back the extremists.

He said: “Our role now, apart from the air strikes, is increasingly going to be on training. In particular, it will mean dealing with car and truck bombs and roadside devices, as well as basic infantry skills.

“We have not finalised numbers yet – obviously we have got a lot of kit back from Afghanistan that we can make available – but we are talking very low hundreds.”

Isis fighters were increasingly tucked away in towns and villages as a result of the air strikes, he said.

“That means they have got to be rooted out by ground troops. This has to be done by an own-grown army, not by western groups.”

The Labour MP Graham Allen said the escalation represented exactly the kind of mission creep opponents of the initial engagement had feared and suggested MPs be asked again to give their approval.

Allen added that he feared that if troop numbers could be significantly raised without parliament’s approval then the extension of operations to Syria could be too – something specifically excluded from the Commons motion approving action to Iraq.

The shadow defence secretary, Vernon Coaker said: “The government should provide clarity about the scale, scope and timeframe of the deployment of these further trainers.

“The defence secretary should clearly outline the numbers of military trainers involved and the work that they will be doing, so that there can be no misunderstanding about the role of British troops in Iraq.”