Also speaking at the Pentagon on Friday, the Chairman of the Joint Chiefs of Staff General Joseph Dunford, said strikes against Syrian government-backed fighters this week were a "force protection measure", and would not happen again if us forces are not threatened.
"If this goes to a military solution, it's going to be tragic on an unbelievable scale", Mattis said.
The chairman of the Joint Chiefs of Staff, Gen. Joseph Dunford, told reporters at the Pentagon that negotiations with Moscow were underway and were aimed at allowing United States and Russian forces to operate in proximity to one another.
It's not yet clear how this change will affect the size and scope of USA troop deployments to Iraq and Syria.
"They know our intent is the foreign fighters do not get out", he said.
Mattis acknowledged that, although US forces don't "close in" on the enemy with local forces, having them closer to the battlefield meant greater risk.
"Thanks to the leadership and authorities granted by President (Donald J.) Trump, thanks to the spirit of dozens of nations committed to this fight, thanks to the nations whose troops have gone toe-to-toe with this terrorist group ... we have retaken over 55 percent of ISIS territory there in the core", Mattis said.
South Korea has said the North's missile program was progressing faster than expected, with Sunday's test considered successful in flight.
"He directed a tactical shift from shoving ISIS out of safe locations in an attrition fight to surrounding the enemy in their strongholds so we can annihilate ISIS", Mattis said.
Lawmakers: Rosenstein Mum On White House Pressure For Comey Memo
The Justice Department on Friday distributed prepared remarks that Rosenstein delivered to Congress in separate briefings. He says his memo is not a finding of official misconduct and is not a statement of reasons to justify Comey's firing.
McGurk said the State and Treasury departments are also working with allies to more rigorously track foreign fighters and investigate ISIS' currency-sharing methods.
"This enemy remains adaptive, and the threat will not end with the battles of Mosul and Raqqa, but the end of the phony caliphate is coming into sight". At the same time, Iraqi government forces, also supported by USA advisers and airpower, would advance toward al-Qaim.
West Mosul is surrounded, as is the city of Tal Afar, he said, naming Ninewa, Hawija, and the western Euphrates River Valley as next targets in Iraq.
Post-Mosul, post-Raqqa, the intent will be to militarily squeeze this stretch of territory from each end, according to USA officials.
The Trump administration is pursuing a policy of "maximum pressure and engagement" to, in the words of Admiral Harry Harris, head of U.S. Pacific Command, "bring Kim Jong Un to his senses", but not necessarily "to his knees".
A member of the US-backed Syrian Democratic Forces, made up of an alliance of Arab and Kurdish fighters, removes an Islamic State group flag in the town of Tabqa, April 30, 2017.
Deir el-Zour presents an especially tricky challenge because the Syrian military has a base there and the USA has avoided tangling with President Bashar Assad's forces other than an April 7 cruise missile strike.
"ISIS will go down fighting, of course, and do enormous amounts of harm in its death throes", Hal Brands and Peter Feaver, two USA defense analysts, wrote in a recent assessment of how the extremist group will be defeated.
While the Middle Euphrates River Valley corridor may be the next key battleground, US officials believe it will not be the last.
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