Speaking to CBS on Sunday, Mattis said the change of policy was primarily aimed at preventing Daesh’s foreign members from returning to their own countries.
"We have already shifted from attrition tactics, where we shove them from one position to another in Iraq and Syria, to annihilation tactics, where we surround them. Our intention is that the foreign fighters do not survive the fight to return home,” Mattis claimed.
"Civilian casualties are a fact of life in this sort of situation,” the Pentagon chief added, claiming that "We’ve had success on the battlefield."
The comments came days after reports emerged that US-led airstrikes against purported Daesh positions in Iraq and Syria had led to heavy civilian casualties on several occasions over the past weeks.
On Thursday, the Pentagon admitted that at least 105 Iraqi civilians, including 42 children, lost their lives after a US Air Force fighter jet targeted a building in the embattled Iraqi city of Mosul back in March.
Fresh airstrikes by the US-led campaign killed at least 20 more Iraqi civilians in the northern parts of Mosul's Old City on Sunday, according to Arabic-language al-Forat news agency.
Also on Sunday, the so-called Syrian Observatory for Human Rights (SOHR) reported that around 20 civilians had been killed in a US airstrike against southern outskirts of the Syrian city of Raqqah.
The US and a number of its allies have been pounding parts of Iraq and Syria since 2014. The campaign has yet to yield any meaningful achievements besides destroying hospitals and other critical infrastructure.
Mattis said Sunday that the US military was still trying to "avoid civilian casualties at all costs,” doing "everything humanly possible consistent with military necessity.”