The U.S. is adding 350 more troops to help protect the American Embassy in Baghdad and its support facilities in the capital, raising the number of U.S. forces in the country to over 1,000, officials said Tuesday.

President Barack Obama approved the additional troops for protection of American personnel following a request by the State Department and a review and recommendation by the Defense Department, the White House said in a statement.

The buildup of U.S. troops in Baghdad follows the growing threat from Islamic State militants in northern Iraq. Since early August the U.S. has carried out 124 airstrikes against the militants, the latest taking place near Mosul Dam on Monday.

The additional troops will not serve in a combat role, the White House said. Most are from the Army and some are Marines, the Pentagon said in a statement.

Approximately 820 troops have now been assigned to augment diplomatic security in Iraq, said Rear Adm. John Kirby, the Pentagon's spokesman.

The additional troops 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, Kirby said. Fifty-five troops in Baghdad since June will be redeployed outside of Iraq and replaced by 405 newly deployed troops, he said.

ISIS accused of 'ethnic cleansing'

Meanwhile, an international rights group has accused ISIS on Tuesday of carrying out a systematic campaign of "ethnic cleansing" in northern Iraq that includes mass killings, abductions and other war crimes.

In a new report, Amnesty International said Islamic State militants have abducted "hundreds, if not thousands" of women and children who belong to the ancient Yazidi faith. The extremists also have rounded up Yazidi men and boys before killing them, the London-based group said.

The 26-page report adds to a growing body of evidence outlining the scope and extent of the Islamic State group's crimes since it began its sweep from Syria across neighbouring Iraq in June. The militants have since seized much of northern and western Iraq, and have stretched as far as the outskirts of the Iraqi capital Baghdad.

Religious, ethnic minorities forced from their homes

On Monday, the United Nations' top human rights body approved a request by Iraq to open an investigation into alleged crimes committed by the Islamic State group against civilians. Its aim is to provide the Human Rights Council with a report and evidence that could shed further light on Iraqi atrocities and be used as part of any international war crimes prosecution.

A girl carries water bottles she received from a Kurdish political party after the siege by ISIS militants was broken in Amerli. (Youssef Boudlal/Reuters) In its report, Amnesty detailed how Islamic State group fighters expelled Christians, Shias, Yazidis and others from their homes. It documented several cases where the militants rounded up Yazidi men and boys and killed them in groups after overrunning their ancestral lands in Iraq's far north.

Most fled as extremists neared their communities, fearing they'd be killed or forcibly converted to the group's hard-line version of Islam.

Thousands of Christians now live in schools and churches in northern Iraq. Yazidis crowd into a displaced persons camp and half-finished buildings. Shias have mostly drifted to southern Iraq.

The sudden displacement of the minority groups appears to be the final blow to the continuity of those tiny communities in Iraq. Their numbers had been shrinking since the U.S.-led invasion of Iraq in 2003, which triggered extremist violence against them.

"Minorities in Iraq have been targeted at different points in the past, but [ISIS] has managed, in the space of a few weeks, to completely wipe off of the map of Iraq, the religious and ethnic minorities from the area under their control," said Donatella Rovera of Amnesty International.

The Yazidis, in particular, were harshly targeted as ISIS militants overran their ancestral lands in August.

In one incident, the report said "possibly hundreds" were killed in the village of Kocho on Aug. 15 after militants told residents to gather in a school.

"They separated men and boys from women and younger children. The men were then bundled into pickup vehicles — some 15-20 in each vehicle — and driven away to different nearby locations, where they were shot," the report said.

Thousands of women possibly held captive

ISIS fighters also systematically seized Yazidi women and children, some as they rounded up villagers, others as they tried to flee the militant onslaught, the report said. Their fate is unclear.

The report said they had obtained the names of "scores of the women and children" seized by the group. It said "hundreds, possibly thousands," were likely being held.

[ISIS] has managed, in the space of a few weeks, to completely wipe off of the map of Iraq, the religious and ethnic minorities from the area under their control. - Donatella Rovera of Amnesty International

Some captive women are secretly communicating with their families on cell phones, Amnesty said. They told their families that some girls and young women were separated and taken away, Amnesty said.

It appears that some teenage girls were taken in groups to the homes of ISIS fighters, the report said.

The brother of one girl who escaped the militants told The Associated Press that his 17-year-old sister was held with another Yazidi teenage girl in a house in the Iraqi town of Falluja. Khairy Sabri said militants threatened to kill his sister Samira if she did not convert to Islam. Sabri said his sister was seized on Aug. 3 and was moved three times.

After fighting intensified between Kurdish forces and the militants, the three ISIS group fighters guarding the house fled, allowing the women to escape, Sabri said. Sabri said his sister was otherwise unharmed.

Amnesty noted allegations that some abducted women were raped or forced to marry fighters.

The group said detained women who were in contact with their families had not been harmed, but "they believe that others have, notably those who were moved to undisclosed locations and have not been heard from since."

Yazidi lawmaker, Mahma Khalil, called on the Iraqi government and international community to urgently help the Yazidis who are still facing "continuing atrocities" by the militants.

"They have been trying hard to force us to abandon our religion. We reject that because we are the oldest faith in Iraq, that has roots in Mesopotamia," Khalil said.