Iraqis fleeing the Islamic State stronghold of Mosul reunited with their families on Friday, holding hands through the wire fence of a refugee camp and lifting up new babies for their relatives to see.

Many broke down in tears as the saw loved ones for the first time in more than two years.

"This is my family, these are my people, I cannot describe the feeling," Ziyad Ezz-Eldin said at the barrier of the Al-Khazer displacement camp, to the east of the city.

"I have not seen them for two years and a half. They left in the beginning of the crisis, but I stayed in Mosul, this was our fate. It was terrible under Daesh (Islamic State)."

Thousands have left Mosul since Iraqi regular troops and special forces, Shi'ite militias, Kurdish peshmerga fighters and other groups backed by US-led air strikes launched their campaign to retake the city nearly three weeks ago.

The camp's inhabitants have to stay there until they have cleared security screening and other checks.

Family members, some of whom said they got out just in time before Islamic State took over Mosul in 2014, pressed outside, impatient for them to be released.

"My mother and father are in Mosul, they stayed there, I left without them. I have not seen them for two years and a half, how can anyone accept this," Abu Zahed said.

"I just want to see them that is all I want in life."

The United Nations says 22,000 people have been displaced since the start of the Mosul offensive, excluding thousands of others from nearby villages forced back to Mosul by retreating Islamic State fighters who used them as human shields.