Washington (AFP) - At least four Western hostages held by the Islamic State in Syria, including murdered American journalist James Foley, were waterboarded in the early part of their captivity, The Washington Post said Thursday.

Foley, whose recent execution at the hands of the extremists provoked revulsion, and the other kidnapped Westerners were waterboarded "several times," the newspaper said, citing people familiar with their treatment.

Sources involved in trying to free the hostages have confirmed to AFP that waterboarding was used on at least one hostage.

Waterboarding, which was used by the CIA during interrogations of suspected terrorists after the September 11, 2001 attacks, is a widely condemned form of torture that simulates drowning.

The Post quoted one person with direct knowledge of what happened to the hostages as saying the Islamists, who last week released a grisly video showing Foley's beheading, "knew exactly how it was done."

The captives, including Foley who was kidnapped in northern Syria in November 2012, were held in Raqa -- the heartland of the "caliphate" IS has declared -- the Post said.

It quoted a second person familiar with Foley's time in captivity as saying that the American, who contributed reports to GlobalPost, Agence France-Presse and other media outlets, was tortured, including by waterboarding.

The black-masked militant seen in the video holding Foley, 40, by the scruff of the neck, said the journalist's killing was to avenge American airstrikes against the IS in Iraq.