BAGHDAD (AFP) - Military operations aimed at retaking the Iraqi city of Tikrit from the Islamic State in Iraq and Syria (ISIS) group have caused around 28,000 people to flee their homes, the United Nations said on Thursday.

"Military operations in and around Tikrit have precipitated displacement of an estimated 28,000 people to Samarra," a UN statement said.

"Field reports indicate that additional displacements are underway and that yet more families remain stuck at checkpoints," it said.

Some 30,000 Iraqi security forces members and allied fighters launched an operation to retake Tikrit on Monday, the largest of its kind since ISIS forces overran swathes of territory last June.

Retaking Tikrit, the hometown of now-executed Iraqi president Saddam Hussein, from militants who have had over eight months to dig in poses a major challenge for the country's forces.

Sectarian-fuelled revenge killings targeting Sunni Arabs have been a feature of past operations involving Shi'ite militias, raising concerns that the same may happen in Tikrit.