RSF urged everyone to step up the campaign for his release and invited Arabic-language news media and bloggers to post a banner on their websites calling for him to be freed.
"We appeal to all Palestinian and Arab media and bloggers to condemn Johnston's kidnapping, which once again highlights the difficulties that foreign journalists working in the Gaza Strip must face," the press freedom organization said. "The silence surrounding this kidnapping is particularly worrying."
"We urge anyone who knows where Alan Johnston is to ensure his safety," said IFJ General Secretary Aidan White. "If he is being held, we are calling on his captors to release him immediately and unconditionally."
The National Union of Journalists for UK and Ireland and the Palestinian Journalists' Syndicate, both affiliated to the IFJ, have also called for Johnston's immediate and safe return.
A total of 14 foreign journalists have been kidnapped in the Gaza Strip since August 2005. Most of them were freed quickly and were not hurt. In most cases the kidnappers used the hostages as bargaining chips to obtain work or to obtain the release of friends held in Palestinian prisons.
Johnston, who is the BBC's main correspondent in Gaza, was kidnapped on 12 March as he was leaving his office to go home. His abductors intercepted his car and forced him to go with them.
No group has claimed responsibility for the abduction, which has been condemned by President Mahmud Abbas, Prime Minister Ismael Haniyeh and the heads of the leading Palestinian political movements.
Nonetheless, government spokesman Ghazi Hamad told journalists that the authorities had information about the kidnappers, although they did not know where Johnston was being held.
Aged 44, Johnston has been covering the Palestinian Territories for the BBC for three years. His colleagues say he took many precautions whenever moving about and had cut back on his movements, especially at night. Some 20 Palestinian journalists gathered outside the parliament on 17 March to demand his release.(EÜ)
