BAGHDAD (AFP) - The United Nations said Sunday (Feb 19) as Iraqi forces launched their offensive to retake west Mosul that it was rushing to build more shelters ahead of an expected wave of displacement.

"We are racing against the clock to prepare emergency sites south of Mosul to receive displaced families," the UN's humanitarian coordinator in Iraq, Lise Grande, said in a statement.

"The humanitarian operation is already stretched. We are trying to reach more than six million people across Iraq who need help. We don't have all of the funding we need and many partners are facing major capacity constraints," she said.

Iraqi federal forces on Sunday launched a new phase in the four-month-old offensive to retake Mosul, the country's second city and the last major stronghold ISIS has in the country.

Forces retook at least five villages and were heading towards the airport, as part of a push aimed at retaking the city's west.

Iraqi government forces last month cleared the eastern side, and while fewer people than feared fled their homes, the UN said a total of 217,000 people have been displaced since the broader Mosul operation started on Oct 17.

It also said a total of 57,000 had already returned to their homes.