The Islamic State militant group is holding hostage a young American woman who was doing humanitarian aid work in Syria, a family representative said Tuesday. The 26-year-old woman is the third American known to have been kidnapped by the militant group.

The Islamic State group recently threatened to kill American hostages to avenge the crushing airstrikes in Iraq against militants advancing on Mount Sinjar and the Kurdish capital of Erbil.

The 26-year-old woman was captured last year while working with three humanitarian groups in Syria. A representative for the family and U.S. officials asked that the woman not be identified out of fear for her safety. All spoke on condition of anonymity because they were not authorized to discuss the issue publicly. 

James Foley

Journalist James Foley was honoured Sunday at a Mass in Rochester, N.H. He was kidnapped and killed by ISIS militants while covering the war in Syria. (Nicole Tung/freejamesfoley.org/Associated Press)

A ransom demand of $6.6 million US has been made, ABC News reported.

More than a week ago, freelance journalist James Foley of Rochester, N.H., was beheaded by the Islamic State group, which kidnapped him in November 2012. Foley, 40, has worked in a number of conflict zones across the Mideast, including Iraq, Libya and Syria. He was in northern Syria on assignment for Agence France-Press and the Boston-based media company GlobalPost when the car he was riding in was stopped by four militants in a contested battle zone that both Sunni rebel fighters and government forces were trying to control.

Another American reportedly killed

The Islamic State video of Foley's beheading also showed another of the missing American journalists, Steven Sotloff, and warned he would be killed next if U.S. airstrikes continued. U.S. officials believe the video was made days before its release and have grown increasingly worried about Sotloff's fate. 

American Freed Syria

Peter Theo Curtis, a U.S. citizen held hostage for about two years by an al-Qaeda linked group in Syria, was released on Sunday. (Associated Press)

Other American hostages have been held by other militant groups, including Peter Curtis from Boston, who was recently released by al-Nusra Front, a rival Sunni extremist group.

Curtis was back on U.S. soil late Tuesday night.

Another U.S. freelance journalist, Austin Tice, disappeared in Syria in August 2012 and is believed to be held by the Syrian government. Tice was working for The Washington Post, McClatchy Newspapers and other media outlets when he was kidnapped. 

Meanwhile, a U.S. official says an American man reported to have been killed in Syria was there to fight alongside a terrorist group, most likely the Islamic State.

The official said Tuesday that investigators were aware that Douglas McArthur McCain of San Diego had traveled to the country to join a militant group.

A relative, Kenneth McCain, said the U.S. State Department had called his family to inform them that Douglas McCain had been killed in Syria.

The official said investigators did not yet have McCain's body and were still trying to verify information about his death.

Jen Psaki, State Department spokeswoman, said U.S. officials are in contact with the family in providing consular assistance. "As you know there is typically a process that has to be gone through before any confirmation can be made and certainly, out of respect for the family, we won't be adding any more comment at this time," she said.

'America does not forget'

The Islamic State militant group is seeking to create a caliphate across parts of Syria and Iraq. The militant group is so ruthless in its attacks against all people they consider heretics or infidels that it has been disowned by al-Qaeda's leaders.

President Barack Obama said in a speech in North Carolina on Tuesday that "America does not forget" and vowed justice for Foley's murder.

In its annual report last November, the New York-based Committee to Protect Journalists estimated at least 30 journalists have been kidnapped or have disappeared in Syria — held and threatened with death by extremists or taken captive by gangs seeking ransom. CPJ described the widespread seizure of journalists as unprecedented, and largely unreported by news organizations in the hope that keeping the kidnappings out of public view may help to negotiate the captives' release.

The group reported 52 journalists have been killed since Syria's civil war began in early 2011, and documented at least 24 other journalists who disappeared earlier this year but are now safe.

Separately, the Paris-based Reporters Without Borders last fall cited higher figures, saying at least 60 "news providers" are being detained and more than 110 have been killed.

With files from CBC