Britain’s defence secretary, Michael Fallon, has promised UK troops will be sent to Iraq only as part of a limited training mission, as Amnesty International called for reassurances that British forces will not be complicit in Baghdad-backed “death squads”.

Speaking from Iraq, Fallon said a small number of officers would be involved in a strategic and advisory role, such as how to defuse roadside bombs, but would not put a number on how many would be sent.

He insisted this did not constitute boots on the ground, as David Cameron has promised not to involve combat troops in anything by airstrikes against Islamic State (Isis).

He said: “This is a very limited mission. The prime minister’s made it very, very clear we are not going to recommit combat troops to Iraq. We’ve been there, we’ve done that.

“What we are going to do is to help the new government of Iraq and its own army take the fight to Isil through the aircraft that we’ve deployed in the sky, through intelligence gathering and through specialist training, particularly in countering IED (improvised explosive devices), the roadside bombs and the car bombs where we have experience to contribute.”

However, Amnesty International warned the UK must be careful not to become implicated in human rights abuse reportedly committed by Baghdad-backed militias.

The head of policy at Amnesty, Allan Hogarth, said: “Iraqi government forces have committed a string of serious human rights violations in recent months and British military trainers in Iraq must ensure they do not become complicit in such abuses.

“In particular, Baghdad-backed Shia militias have been acting as little more than death squads, abducting, torturing and killing those they believe are Isis supporters.

“British forces in Iraq should seek to act as a brake on the terrible cycle of human rights abuse unfolding in the country, including by ensuring that everyone they train is fully aware of the Geneva conventions and the responsibility of all armed forces to act accordingly.”

Fallon also promised that Britain would do more to train and support to Kurdish peshmerga forces in their fight against Isis militants in northern Iraq. During a visit to a base on the outskirts of the Kurdish capital, Fallon described the fight against Isis as Britain’s fight and announced that 40 Browning heavy machine guns supplied by the UK would now be used on the frontline after the peshmerga forces completed their training.

The Kurds have emerged as the west’s most trusted partners on the ground in the war against the extremist Isis group in Iraq and Syria. In recent weeks, Kurdish peshmerga forces have been able to regain ground from Isis in northern Iraq with the help of coalition airstrikes, including British Tornado jets.

Fallon said: “This is not the end of our support, we are announcing today further training and further support. We are offering some small unit training in infantry tactics, in sharp-shooting and in battlefield first aid. Further to this, we will be making further gifts of supplies, spare parts for the machine guns, binoculars and first aid equipment.”

After Isis made their push towards Kurdish areas in early August, western countries have been sending weapons and ammunition to Kurdish forces in Iraq. Since September, the Ministry of Defence has provided 40 heavy machine guns, bomb disposal training, help with intelligence-gathering, and transporting weapons and ammunition to Kurdistan from other western countries.

Bryar Najar, a 24-year-old peshmerga who recently fought Isis near Mosul dam and a town on the Iraq-Syria border said he was impressed with the professionalism of the British forces and grateful for the assistance provided by the government. “This training is very important for the peshmerga to fight Daesh [Isis]. We have been training with this new gun for two weeks and I feel confident to teach others how to use this heavy gun,” he said.

Another peshmerga, Salim Sediq, who fought in the town of Gwer, south-west of Erbil, said: “The old guns we have do not pierce Humvees, but this British one can pierce armoured vehicles. On one occasion in Gwer, 18 Isis Humvees attacked us and we had no guns that could destroy or hold back the Humvees and as a result we had to pull back.”

Brigadier General Qaraman Kamal said the British trainers have trained more than 300 peshmerga fighters to use the Browning heavy machine guns, which will be very effective against Isis. “We have asked the British government to send us more heavy weaponry to destroy Daesh. We do not want British or American troops to come and fight, we can do the fighting – all we need is training and weapons.”

The move comes days after the departure of the last UK troops from Afghanistan and three years after their withdrawal from Iraq.

The prime minister, David Cameron, has promised there will be no troops on the ground fighting Isis, but the UK has been taking part in military air strikes over Iraq. A small number of personnel have been helping to train Kurdish peshmerga forces.

The UK has been coming under international pressure to join the US in providing more military advice to the new Iraqi prime minister, Haider al-Abadi, as the country’s forces are struggling to combat Isis in the north and west.

However, Amnesty has accused the Iraqi government of arming and supporting Shia militias who are killing Sunni civilians with impunity, without evidence they are connected to Isis.

Abadi, who took office in September, has admitted to previous “excesses” by security forces and promised reforms for the whole country.