"Can every Muslim and weak person find refuge in this caliphate? Or would it be like a sharp sword against all opponents?" Issam Barqawi, known as Abu Mohammed al-Maqdessi, wrote on Facebook and on jihadist websites.
On Sunday, militants previously known as the Islamic State of Iraq and the Levant (ISIL), declared a "caliphate", an Islamic form of government last seen under the Ottoman Empire, straddling parts of Iraq and Syria.
The militants, who renaming themselves the Islamic State (IS), already control large swathes of territory in north and east Syria, and this month captured vast stretches of northern and western Iraq.
"What would the fate be of other Islamist fighters in Iraq, Syria and elsewhere?" asked Maqdessi, who was freed on July 16 after serving a jail sentence for recruiting fighters for the Taliban.
Once mentor to Iraq's now slain Al-Qaeda leader Abu Musab al-Zarqawi, before the two fell out over ideological differences, Maqdessi warned against "Muslims who kill other Muslims".
"Do not think you can silence the voice of justice by shouting, making threats, aggression and having no manners," he said.
Experts say the declaration of the caliphate is a direct challenge to Al-Qaeda and could spark a contest for the leadership of the global extremist group.
Jordan's jihadist movement is generally dominated by anti-IS groups that support Al-Qaeda and its Syrian ally, Al-Nusra Front.
The offensive in Iraq has also sparked fears in Amman that the Sunni militants will try to take their fight to the kingdom.
Jordan is already suffering from hosting more than 600,000 Syrian refugees, and has long faced the challenge of dealing with its own Islamist extremists, many of whom have joined jihadists or Al-Qaeda-linked groups in neighbouring Iraq and Syria.
On Monday, King Abdullah II appealed for international support to help Jordan deal with regional turmoil after the caliphate was declared.