"I address Daesh (ISIS) and tell them that you have no choice but to surrender or get killed," al-Abadi said.
Tal Afar is the last city still under the control of ISIS militants in Iraq's Nineveh province following the liberation of Mosul
Al-Abadi said Iraqi Security Forces (ISF) counter-terrorism units, militia, federal and local police are taking part in the operation. They will be backed by air power from international coalition forces, with engineering and medical support on hand.
He said Arabs, Kurds, Turkmen, Muslims, Christians and Yezidis, which he described as "first-class citizens," are taking part in the assault.
"Following their historic victory in Mosul, the ISF have proven themselves a capable, formidable, and increasingly professional force, and they are well prepared to deliver another defeat to ISIS in Tal Afar, like Mosul," the US-led international coalition said in a statement.
"Mosul was a decisive victory for the Iraqi Security Forces, but it did not mark the end of (ISIS) in Iraq or its worldwide threat," said US Army Lt. Gen. Stephen Townsend, the commander of US and coalition forces in Iraq and Syria.
Thousands of civilians flee
The coalition estimates that 10,000-50,000 civilians remain in and around Tal Afar.
In a statement issued on Saturday, the United Nations said thousands of civilians are now fleeing the area for safety.
"Families are trekking for 10 to 20 hours in extreme heat to reach mustering points. They are arriving exhausted and dehydrated," said Lise Grande, the UN's humanitarian coordinator for Iraq.
"More than 30,000 people have already fled the district," she said. "We don't know how many civilians are still in the areas where fighting is occurring, but we are preparing for thousands more to flee in coming days and weeks. Conditions are very tough in the city. Food and water are running out, and people lack the basic necessities to survive."
She called on the warring parties "to avoid civilian casualties and ensure people have the assistance they are entitled to under international humanitarian law."
Yazidi children still held captive
Advocates for the Yazidi people, a Kurdish religious minority persecuted by ISIS, say many Yazidi children are held captive in Tal Afar, their names and identities changed by the jihadists.
In a tweet, the Kurdish news agency Rudaw said two Yazidi girls had already been rescued.
Along with Tal Afar, ISIS fighters still control Hawija west of Kirkuk, as well as the towns of Qaim, Rawa and Ana near the Syrian border.
ISIS captured Tal Afar on June 16, 2014, after a two-day battle.