This image posted on an Islamic website on June 15, 2004 shows blindfolded US hostage Paul Johnson being held in Saudi Arabia. Al Qaeda militants beheaded the American engineer June 18, 2004. (Reuters photo)

A statement purportedly from al-Qaida said Tuesday that the group would kill an American hostage unless its militants jailed in Saudi Arabia were released within 72 hours.

"If the tyrants of the Saudi government want the US hostage Paul Marshall Johnson to be released, they must release our mujahedeen held in prisons in Haer, Ar-Ruwais and Allecha within 72 hours... otherwise, we will execute him," said the statement carried by website www.hostinganime.com/sout18.

The statement was signed by the Al-Qaida network in the Arabian peninsula.

Johnson, a 49-year-old chief engineer working for top US defense contractor Lockheed Martin, has been missing in Saudi Arabia since Saturday.

His working card and two pictures were posted on the website.

June 18 is the deadline set by the terrorist group for the Saudi government to satisfy its demand.

Saudi Arabia has killed or arrested dozens of terror suspects in its crackdown on al-Qaida, which has claimed responsibility for a series of deadly attacks targeting Westerners in the kingdom.