Imams involved in the nationwide anti-terror campaign are instead encouraging donations to Syrian cause through legitimate charities.
Their message coincides with the holy month of Ramadan, which began this weekend.
It comes after a video emerged showing two young Cardiff men urging others to join the jihadist fight in Syria - where tens of thousands have died in three years of war - and in Iraq.
These men are believed to be among the 500 Britons fighting in Syria for the militant group ISIS (Islamic State in Iraq and the Levant).
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Muthana's younger brother Aseel Muthana (17) has also reportedly joined them.
Earlier this week, their father had criticised the British government's anti-radicalisation strategy in Cardiff, saying it failed as it was made up of mainly white officers.
Sheikh Zane Abdo from the South Wales Islamic Centre, the local imam in Wales, has said he believed the men had been "groomed" by people they had met outside of their mosque.
"These are young men who are very sincere, they want to do something good. But you can be sincere and you can be sincerely deluded in what you want to do, and they have been groomed to think a particular way," he said.