President Obama approved sending 350 additional troops to the Iraqi capital to increase security at the Baghdad embassy compound and its support facilities.
The Pentagon said late Tuesday that the new personnel will build upon previous deployments that were announced in June, boosting security in Iraq to about 820 people.
The forces are expected to provide security for the State Department, as the U.S. military continues to carry out airstrikes on Islamic State militants in Iraq.
"The additional joint forces will come from within the U.S. Central Command area of operations and will include a headquarters element, medical personnel, associated helicopters, and an air liaison team," Pentagon spokesman Rear Adm. John Kirby said in a statement.
The White House stressed that the additional forces will not serve in a combat role.
Since the Pentagon began the air campaign Aug. 8, the military said it has conducted 124 strikes on Islamic State targets.
In mid-June, there were just 200 to 300 personnel in Iraq. As Islamic State seized Mosul and began to advance toward Baghdad, the administration began sending more people to the country.
With the 350 new arrivals, there will be 1,113 personnel in Iraq.
The announcement of increasing security forces comes the same day that Islamic State posted a video to the Internet that purportedly shows the beheading of American journalist Steven Sotloff. The video has yet to be verified as authentic.
"The Department of Defense will continue to plan and prepare further military options should they become necessary, and we will remain ready to protect our diplomats, our citizens, and our interests in Iraq, while we continue to work with the Iraqi government to counter the Islamic State," Kirby said.
Josh Earnest, White House press secretary, reiterated Obama's pledge to protect American interests and his call for regional allies and partners in the Middle East to help take on Islamic State militants.
Obama is among about 60 global leaders converging on Cardiff, Wales, this week for a NATO summit.
"The president will be consulting this week with NATO allies regarding additional actions to take against [Islamic State] and to develop a broad-based international coalition," he said. "Secretary Kerry, Secretary Hagel, and President Obama's counter-terrorism advisor, Lisa Monaco, will be traveling separately to the region in the near term to build a stronger regional partnership."
Times staff writer Christi Parsons contributed to this report.