An Islamist website has shown what appears to be the beheading in Iraq of a man described as a CIA agent.
One recent video turned out to be a hoax filmed in a California garage
In the four-minute video, a Western-looking man was seen surrounded by masked men, one of whom seemed to sever the victim's head with a long knife.
US officials said they were trying to establish whether the tape was genuine.
A video posted on an Islamic militant website on Saturday apparently showing an American being beheaded was later found to be a hoax.
The latest video was shown as the Bulgarian authorities confirmed that a headless body found in the River Tigris in Iraq was that of kidnapped lorry-driver Ivaylo Kepov.
Group not identified
In the video, the apparent victim was shown seated wearing a plain T-shirt and trousers.
A sign round his neck read "CIA agent" and the Arabic word for "visitor".
However, the Associated Press news agency quoted a US official as saying the CIA had accounted for all its employees.
The website did not say which group had issued the video.
The masked men did not say who they were or make any demands, as in some previous videos.
One of the men was seen in close-up cutting at the man's throat, then holding the head in the air as voices were heard shouting "Allah is great".
Dozens of people have been kidnapped by armed groups in Iraq, and at least nine have been killed.
There are currently three Americans - a soldier and two civilian contractors - known to be missing in Iraq.
Private Keith Maupin was reported to have been taken hostage in April and shot dead by his captors in June, but his death has never been verified.
The other two, William Bradley and Timothy Bell, went missing at the same time as Pfc Maupin but have not been seen since.