A shadowy group holding a British correspondent for three months threatened on Sunday to kill him.
A spokesman for The Army of Islam, identified as Abu Khattab, told Al-Jazeera that there was no deal to release BBC correspondent Alan Johnston, and " if the situation gets more complicated concerning us and our group, then we will ingratiate ourselves to Allah by slaying this journalist."
The authenticity of the video could not be independently confirmed. Earlier, a Hamas spokesman in Gaza said there had been no new developments in the case, despite reports that Johnston would be released soon.
Johnston was abducted from a Gaza City street on March 12.
The deposed Palestinian prime minister, Ismail Haniyeh, said earlier that his Hamas movement is moving in a "serious, active way" to win the release Johnston.
After taking control of the Gaza Strip last week, Hamas officials said they had made contact with Johnston's kidnappers and would work to win his release - in an apparent bid to gain favor with the international community and impose order in chaotic Gaza. Since then, the group has sent mixed messages about how soon Johnston could be freed.
At a news conference, Haniyeh said Hamas was doing its utmost to free Johnston, but gave no indication when that might happen.
"We are moving in a serious, active way and I hope, God willing, that this issue will soon end," Haniyeh said. "We are all concerned ... for the safety and security of Alan Johnston."
Johnstonwas snatched in Gaza by a group believed to have some links to Hamas, and a message purporting to be from his captors has demanded the release of Islamic prisoners, including a cleric being held in Britain.
Johnston, 45, was seen for the first since his abduction in a video posted two weeks ago on a Web site used by Islamic militants. He appeared calm and said he was being well-treated and was in good health.
His disappearance is the longest of any Western journalist abducted in Gaza and has sparked numerous protests and solidarity marches in London and the Palestinian territories.
Earlier Sunday, Islam Shahwan, a spokesman for Hamas' militia, said Johnston would be freed "very soon." But Sami Abu Zuhri, another Hamas official, played down hopes for his quick release, saying, "there is nothing new yet regarding Alan."
Palestinian officials have said they know where to find Johnston, but have held back on raiding the hideout at Britain's request, for fear of harming him.