Fighting in oven-like temperatures, Iraqi tanks and armored vehicles stormed across the desert and Iraqi special forces moving in from the northeast breached a thick city wall, US and Iraqi officials said.
The US and more than 60 of its allies have been carrying out an aerial campaign against what they say are Daesh targets in Iraq and Syria for several years, without uprooting the terrorist group.
The government Sunday launched its military operation to retake Tal Afar, one of the last Iraqi areas held by IS, following the July liberation of Mosul about 80 kilometers to the east.
Both Mattis and the commander of the USA -led coalition to defeat ISIS, Lt. Gen. Stephen Townsend, pointed to the tendency of the Sunni jihadists to use civilians as human shields, as the battle to dislodge the group from its Syrian "capital" continues into its third month.
Meanwhile, between 10,000 and 40,000 civilians still reside in Tal Afar and surrounding areas, according to United Nations estimates. Gen. Andrew Croft, said that over the past few months, the Islamic State has lost much of its ability to command and control its forces. USA officials, speaking on the condition of anonymity, said Mattis, who will meet Prime Minister Haider al-Abadi and Defence Minister Arfan al-Hayali, will discuss the future of U.S. forces in Iraq after the recapture of the remaining cities under Islamic State, and the role they could play in stabilizing operations. "And their cruelty, though, continues to shine through as they hide among women and children and show no regard for non-combatants".
Yet that comeback helped restore the confidence of the Iraqi forces after their shock loss of Mosul and swathes of northern Iraq to IS in 2014, he added.
Tal Afar is the last stronghold in Nineveh Province under IS control.
A key issue is a plan for an independence referendum on September 25 in Iraqi Kurdistan, where Mattis later Tuesday met the autonomous region's president Massud Barzani.
"We're looking very closely at the future beyond [ISIS] and we are encouraged by the leadership of Prime Minister Abadi with this regional engagement", McGurk said. The US official, speaking on the condition of anonymity, said that the memorandum of understanding would expire soon and suggested that Mattis could use it as a bargaining chip. The Pentagon chief told reporters before he left neighbouring Jordan that the Middle Euphrates River Valley - roughly from the western Iraqi city of al-Qaim to the eastern Syrian city of Der el-Zour - will be liberated in time, as IS takes hit from both ends of the valley that bisects Iraq and Syria.