A prominent Saudi cleric has joined in calls to spare the life of a US hostage in Saudi Arabia as the deadline set by his captors to kill him looms.
Mr Johnson's wife pleaded for his release on Arabic TV
Paul Johnson, 49, was seized in the Saudi capital, Riyadh, last Saturday.
The group said on Tuesday he would be killed within 72 hours if al-Qaeda members in Saudi jails were not freed.
Sheikh Saleh bin Abdullah bin Humaid told worshippers at the Great Mosque in Mecca on Friday that hostage-taking and murder were grave sins under Islam.
"Killing a soul without justification is one of the gravest sins under Islam, it is as bad as polytheism," the
state-appointed cleric said at Islam's holiest shrine.
"Whoever kills any person under our protection will not go to heaven. The blood of people under our protection is forbidden... they are on a par with Muslims," he added.
Saudi authorities stepped up their search for Mr Johnson on Friday.
Several thousand Saudi officers swept through Riyadh, going from door to door in some areas. The desert outskirts of the city were also being searched.
Twenty FBI agents specialising in hostage rescue were working with Saudi officers, a Saudi official told the Associated Press news agency.
12 June - US national gunned down in al-Malaz district
8 June - American working for a US defence contractor killed in al-Khalij district
6 June - BBC cameraman killed and BBC reporter seriously injured in al-Suwaydi suburb
Mr Johnson's Thai wife, Thanom, pleaded for his release on Arabic TV. "I want him to come back to me... He didn't do anything wrong," she told the Saudi-owned al-Arabiya satellite channel.
Another leading Saudi cleric, preacher of the capital's Imam Sultana Mosque, urged the kidnappers to release their hostage in an article in the al-Riyadh newspaper on Friday.
They had "trodden the wrong path" and should "come back to the fold of the community of Islam", Sheikh Mohammed bin Saad al-Saeed wrote.
Amid concern over Mr Johnson's fate, the US state department repeated its advice to US citizens to leave after a series of attacks in the kingdom, where three Westerners were killed within a week.
There was credible information that extremists were planning further action against US and Western interests, the department said.
A video from a group calling itself the "al-Qaeda Organisation in the Arabian Peninsula" appeared on an Islamist website on Tuesday, apparently showing Mr Johnson blindfolded.
A man identified as Mr Johnson, who works as a helicopter engineer for US defence contractor Lockheed Martin, is heard giving his name.
The video contained a threat to kill him if the Saudi authorities did not free jailed militants, but did not specify exactly when the deadline set for Friday expires.
A video apparently showing Johnson was posted on an Islamist website
Both the Saudi and US governments said they would do what they could to free Mr Johnson, but vowed not to cave in to the militants' demands.
In Eagleswood, New Jersey, where Mr Johnson grew up, some 100 friends and relatives held a candlelit vigil for him overnight on Thursday.
A similar vigil was held in Brevard County in Florida, where Mr Johnson worked in the early 1980s, while in the family's hometown of Little Egg Harbor, New Jersey, yellow ribbons and support for the Johnsons are on display.
The family made emotional pleas for Mr Johnson's safe return.
"Father's Day is right here. Bring my father home for Father's Day," his son, also called Paul, urged US and Saudi officials.