The lightning offensive by Islamist Hayat Tahrir al-Sham (HTS) group and allied factions saw government forces lose full control of Syria's second city Aleppo for the first time since the civil war began in 2011.

The attacks have caused unease in Iraq, which still bears the scars of decades of conflict, including the rise of jihadists from the Islamic State (IS) group.

"Any infiltration on the Syrian-Iraqi border is absolutely impossible, because of the fortifications and the combat units located there," interior ministry spokesman General Moqdad Miri said on Monday.

The defence ministry said "armoured units of the Iraqi army" were sent to reinforce the border, from the western border town of Al-Qaim to the border with Jordan further south.

Similar forces were deployed along the border further north in Nineveh province, it said.

The move comes after the Syrian Observatory for Human Rights reported 200 fighters from a pro-Iran Iraqi armed group being sent into Syria to support government forces.

The Britain-based war monitor, which has a wide network of sources inside Syria, said the militants entered the Boukamal region through the border at Al-Qaim in two waves.

When contacted by AFP, officials from Iraqi armed factions Kataib Hezbollah, Al-Nujaba and Kataeb Sayyid al-Shuhada denied sending reinforcements.

"It is still too early to take this type of decision," a Kataib Hezbollah commander told AFP, speaking on condition of anonymity.

IS overran large swathes of Iraq and Syria in 2014, proclaiming a "caliphate".

The group was defeated in Iraq in 2017 by local forces backed by a US-led international military coalition.

"Iraq has taken solid precautions after the bitter experience of 2014," Qais al-Mohamadawi, Iraq's deputy commander of joint operations, said on Friday.