WASHINGTON: The Islamic State militant group is holding hostage a young American woman who was doing humanitarian aid work in Syria, a family representative said.

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 airstrikes in Iraq against militants advancing on Mount Sinjar and the Kurdish capital of Irbil.

The 26-year-old woman was captured last year while working with three humanitarian groups in Syria.

A representative for the family and US officials asked Tuesday that the woman not be identified out of fear for her safety.

All spoke on condition of anonymity because they were not authorised to discuss the issue publicly.

More than a week ago, freelance journalist James Foley of Rochester, New Hampshire, was beheaded by the Islamic State group, which kidnapped him in November 2012.

Foley, 40, had worked in a number of conflict zones across the Middle east, including Iraq, Libya and Syria.

He was in northern Syria on assignment for Agence France-Press and the Boston-based news organisation GlobalPost when the car he was riding in was stopped by four militants in a contested battle zone that both rebel fighters and government forces were trying to control.

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 US airstrikes continued.

US officials believe the video was made days before its release and have grown increasingly worried about Sotloff's fate.

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.

Another US freelance journalist, Austin Tice of Houston, 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. 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.

The 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.