If captured BBC journalist Alan Johnston does not walk away as a free man by the end of the day Monday, Hamas will not hesitate to "use all means" to wrest him back safely, the Palestinian Islamist movement has said.

Mahmoud al-Zahar, a senior Hamas official, issued the ultimatum on Monday, warning the Army of Islam group that "the last appointment given for the kidnappers is today."

"If he is not released, we will use all means to secure his life and to free him," al-Zahar told Reuters, adding, "I am looking forward to seeing him."

Johnston, 45, was the only Western journalist living in Gaza when he was abducted March 12 by gunmen suspected to be members of the Army of Islam. The shadowy group appears to draw inspiration from al-Qaeda, and has been linked to violent clan rivalries among Gaza's 1.5 million people.

The intensified demands for Johnston's freedom came the day after a purported spokesman for the Army of Islam threatened that the group would "ingrate ourselves to Allah by slaying this journalist" if the situation becomes more "complicated concerning our group."

In previous web recordings, the Army of Islam has demanded that Britain release Muslim prisoners, including radical Islamist Abu Qatada. However, the British government has refused to submit to any such deal.

The U.K. government responded to reports about al-Zahar's hawkish remarks on Monday with caution and called the latest development "disturbing."

Hamas's apparent bid to win Johnston's release has been interpreted by some analysts as a move to try to win the favour of the international community. The world watched last week as chaos erupted in the Gaza Strip, with Hamas violently taking control of the region from its Western-backed rivals, Fatah.

Foreign Office Minister Kim Howells said she hoped Hamas was not "using this as some sort of publicity stunt to win favour with some elements in the West," BBC News reported. She added that she hoped the standoff could be resolved without the use of force.

Hamas said on Friday that it was in contact with Johnston's kidnappers and was attempting to negotiate his release.