US forces killed Abu Sayed, emir of ISIS-K, Pentagon says U.S. forces killed emir of ISIS in Afghanistan on Tuesday.

 -- U.S. forces killed Abu Sayed, the emir of ISIS-K, in a July 11 strike on the group's headquarters in Afghanistan, the Pentagon said Friday.

ISIS-K, or ISIS-Khorasan, is the Afghanistan branch of the terror group operating in the Khorasan region of the country. The group’s headquarters are located in the Kunar Province.

Defense Secretary James Mattis told reporters on Friday that Sayed’s death was a “victory on the [American] side in terms of setting them [ISIS] back.”

“Significance is, you kill a leader of one of these groups and it sets them back for a day, a week, a month, depending on who it is, what kind of people are below them…[This] is the right direction,” Mattis said.

Chief Pentagon spokesperson Dana White said the strike also killed other ISIS-K members and “will significantly disrupt the terror group’s plans to expand its presence in Afghanistan.”

ISIS chose Abu Sayed to lead the group as its "third emir" after U.S. and Afghan forces killed previous two emirs, Hafiz Sayed Khan in late July 2016 and Abdul Hasib in late April.

U.S. Forces-Afghanistan Commander General John W. Nicholson called Sayed's death "another success" in the campaign against ISIS-K.

“Abu Sayed is the third ISIS-K emir we have killed in the last year, and we will continue until they are annihilated. There is no safe haven for ISIS-K in Afghanistan,” tweeted Nicholson.

ISIS has begun to lose many of its stronghold territories in Iraq, Syria and Afghanistan.

The Iraqi Army declared a “total victory” over ISIS in Mosul on Monday, three years after the group took control of Iraq's second largest city.

The U.S. currently has 8,400 troops in Afghanistan, training and advising the Afghan military in the fight against the Taliban and the ISIS affiliate in Afghanistan. Mattis has indicated a possibility of sending a few more thousand troops to the region, with a review of the U.S. policy in Afghanistan expected mid-July.

Conor Finnegan contributed to this report.