Al Qaeda's Yemen branch published a video purporting to show an American hostage and threatened to kill him if unspecified demands were not met.
In the video, the man identified himself as Luke Somers and said he had been kidnapped well over a year ago. He was looking for "any help that can get me out of this situation".
Reuters was unable to confirm the authenticity of the video, which was posted on YouTube and social media late on Wednesday and carried by SITE, an organization that monitors militant statements.
The man in the video says he was born in the United Kingdom and holds American citizenship.
Somers, a 33-year-old journalist, was kidnapped in Yemen's capital Sanaa in September 2013, joining several other foreigners including Westerners held by militant Sunni Muslim armed groups in the volatile Arabian peninsula country.
In the video, a member of al Qaeda in the Arabian Peninsula (AQAP), the militant network's Yemen arm, criticized the foreign policy of U.S. President Barack Obama which it said had led to deaths and "massacres," mentioning drone strikes in Yemen and air attacks against suspected militants across the Muslim world.
"We warn Obama and the American government of the consequences of proceeding ahead in any other foolish action,” an AQAP official identified as Nasser bin Ali al-Ansi said.
"We give the American government a timeframe of three days from the issuance of this statement to meet our demands about which they are aware; otherwise, the American hostage held by us will meet his inevitable fate," he added, without specifying the demands which he said the United States "knows well".
Ansi also criticized a raid last week by Yemeni and U.S. forces that targeted an AQAP hideout where a number of foreign hostages were being held.
In the assault on a cave in remote Hajr as-Say'ar district in the eastern province of Hadramout, Yemeni security forces rescued six Yemenis, a Saudi and an Ethiopian, and killed seven al Qaeda kidnappers, Yemeni officials have said.
The defense ministry's 26sept.net website later quoted a soldier who had participated in the rescue as saying an American, a Briton and a South African held there had been moved elsewhere two days earlier.
The website had no word on the identity of the three.
U.S. officials say al Qaeda in the Arabian Peninsula (AQAP) has funded its operations with millions of dollars in ransoms received for European hostages.
(Reporting by William Maclean, Editing by Andrew Heavens)