Details have emerged of another American hostage being held by extremist Sunni militants the Islamic State, even as the United Nations accused the group of committing a litany of war crimes and crimes against humanity in Syria.

The Islamic State (formerly known as ISIS) is holding a 26-year-old US woman who was snatched last year while doing aid work in Aleppo, according to a family representative cited by ABC News. The woman's family has asked that she is not named but said that, in exchange for her release, the militants have demanded a $6.6 million ransom and that attempted killer Aafia Siddiqui is released from jail in the US. Siddiqui was convicted in 2010 of attempting to murder Americans serving in Afghanistan and their Afghan colleagues and is currently serving an 86-year sentence.

The female hostage is one of at least three US nationals known to have been held by the Islamic State. Journalist James Foley was murdered by the group in a video which was released online last week. The same footage showed another journalist, Steven Sotloff, alive but in captivity. The militants recently threatened to kill American hostages in revenge for airstrikes on its positions in northern Iraq.

Sotloff's mother Shirley made a personal appeal to the Islamic State's leader Abu Bakr al-Baghdadi today. In a video passed to the New York Times, she says: "I ask you to use your authority to spare his life and to follow the example of the Prophet Muhammad, who protected people of the book. I want what every mother wants; to live to see her children's children. I plead for you to grant me this."

Many, many more are suffering under the Islamic State, according to a report by the UN Independent International Commission of Inquiry released today. This accuses the group's members of war crimes and crimes against humanity in parts of Syria it controls. These include public executions, amputations, lashings, and the use of child soldiers.

"Executions in public spaces have become a common spectacle on Fridays in Ar Raqqah and ISIS-controlled areas of Aleppo governorate. ISIS informs residents beforehand and encourages them to attend. In some cases, ISIS forces passers-by to attend. Children have been present at the executions, which take the form of beheading or shooting in the head at close range," the report says. "Before executions, ISIS fighters announce the victims' crimes. Bodies are placed on public display, often on crucifixes, for up to three days, serving as a warning to local residents."

Children as young as 10 are also being recruited in the Islamic State stronghold of Raqaa, Syria, and given military training, while journalists and other members of the media are specifically and deliberately targeted.

"Members of ISIS have committed war crimes and crimes against humanity in Aleppo and Ar-Raqqah governorates including acts of torture, murder, enforced disappearances and forcible displacement," said Paulo Pinheiro, who chaired the panel which produced the report. "ISIS poses a clear and present danger to civilians, and particularly minorities, under its control in Syria and in the region."

Also today, photographs of the group's capture of the strategically important Tabaqa air base were posted online. One picture shows a group of Islamic State militants shooting seven men — said to be a group of soldiers guarding the base — kneeling in front of them.

Soldiers and officers of the Nosairi regime who were held captive in the battle of Tabaqa Military airport. Image via VK/W_Raqqa.

Other parties in Syria have committed human rights violations too, the report says. This included Syrian President Bashar al-Assad's government, which the commission found was committing war crimes and crimes against humanity with impunity. It specifically accused the Syrian regime of using chemical agents, likely to be chlorine, eight times in western Syria during April and May.

Much of the other government-perpetrated violence involved indiscriminate fire of improvised barrel bombs and missiles in civilian areas, and in some cases, the report says, specifically targeting civilian gatherings. It added that government troops had kept wounded civilians from medical attention, blocked aid from being delivered, and subjected detainees in prison to horrendous torture and sexual assault.

Other rebel groups, which in some cases are fighting both the Islamic State and government forces, have also committed summary executions and massacres, the report says, including attacking civilian areas with car bombs and shelling.

It adds that the impact of the Syrian war is no longer confined to within the country's borders, partly as a result of the success of extremist groups and a constant influx of foreign jihadis.

One of which, an American, fighting with the Islamic State, was killed in Syria over the weekend, according to reports that emerged on Tuesday. The man, identified as Douglas McAuthur McCain attended high school in New Hope, Minnesota, according to his Facebook profile.

Meanwhile, Australian intelligence chief David Irvine said 15 Australians are thought to have died fighting in Syria and Iraq. He added that the Australian Security Intelligence Organization believed around 60 Australians had joined either the Islamic State or al Qaeda's Syrian offshoot Jabhat al-Nusra, AFP reported.

Possibly in response to the US hostage situation, America has now begun surveillance flights over Syria, possibly as a prelude to airstrikes.

Officials told AP that president Barack Obama had approved the flights on Monday and that they had began by Tuesday. Obama has so far steered clear of approving military action in Syria, but should strikes be made, additional intelligence would likely be necessary beforehand.

The US began an ongoing series of strikes against the Islamic State in Iraq earlier this month.