The Sunni militant offensive in Iraq has sparked fears in Amman that they will take the fight to Jordan, which is already struggling with its own home-grown Islamists.

The Islamic State of Iraq and the Levant (ISIL), a powerful jihadist group that controls key areas of Syria, has seized a swathe of Iraqi territory in a lightning offensive Baghdad's forces seem powerless to halt.

Experts fear Jordan could be one of ISIL's next targets, with YouTube videos purporting to show the group threatening to take jihad there.

Already reeling under the impact of hosting more than 600,000 Syrian refugees, Jordan has long faced security challenges in dealing with its own jihadists, many of whom have joined ISIL and other Al-Qaeda-linked groups in Iraq and Syria.

"Only the ignorant or those in denial would think that ISIL does not have supporters in Jordan. How do they explain the presence of 2,000 Jordanian jihadists in Syria and Iraq?" Oraib Rantawi, head of Amman's Al-Quds Centre for Political Studies, told AFP.

On Friday, Islamists demonstrated in the restive southern city of Maan, a hotbed of Islamism, praising ISIL "victories" in Iraq, sources said.

"Jordan needs to worry a lot. ISIL is very organised and strong. The group plans to extend its circle to target and include the kingdom," Hassan Abu Hanieh, an expert on Islamist groups, told AFP.

US President Barack Obama has warned the militant offensive could spread.

"That could spill over into some of our allies like Jordan," he said in an interview Sunday on CBS television's "Face the Nation".

One recent YouTube video shows an ISIL fighter tearing up a Jordanian passport.

"I have a message to the tyrant of Jordan: we are coming to you with death and explosive belts," he said.

Jordan's jihadist movement is generally dominated by anti-ISIL groups that support Al-Qaeda and its Syrian ally Al-Nusra Front, ISIL's rival.

Jihadist ideologues and Salafist leaders in the country have joined Al-Qaeda chief Ayman al-Zawahiri in denouncing ISIL brutality.

Last week, six suspected ISIL followers attacked Iyad al-Quneibi, an outspoken critic of the group, and smashed his car, highlighting inter-Islamist rivalries.

The university professor, seen as pro-Nusra, did not blame anyone, but Salafist leaders and others pointed the finger at ISIL.

"We are not in touch with ISIL, but there is no doubt it has many backers in Jordan," Mohammad Shalabi, a Jordanian Salafist leader better known as Abu Sayyaf, told AFP from jail.

- 'Happy when regimes fall' -

"We are happy to see the tyrannical regimes in Iraq and Syria fall.

"In Iraq, it is a fight between good and evil, and we pray to God that any issues will vanish, paving the way for the creation of Islamic Caliphate."

Mohammed Abu Rumman, a researcher at the University of Jordan's Centre for Strategic Studies, said ISIL support is growing.

"It is dangerous and this should make the Jordanian authorities concerned," he told AFP.

"These developments impose big security and strategic challenges on Jordan. It is a nightmare."

According to global intelligence firm Stratfor, the ISIL's "intent to expand into Jordan follows the region's geopolitical logic".

"After its push into Iraq, and already controlling significant swathes of Syrian territory, the jihadist group can try to push into the kingdom... Jordan is the only opening available to ISIL," Stratfor said on its website.

But ISIL still faces considerable challenges, it added.

"The Jordanian regime is far more stable than Syria or Iraq, and its security forces have proved to be quite effective. Furthermore, Jordan has strong backing from the United States and Saudi Arabia."

Jordan takes such warnings seriously.

"The Jordanian army has dispatched more troops, tanks, rocket launchers and armoured personnel carriers to the border with Iraq," a security official told AFP on Monday, after militants have seized the Al-Waleed border post between Iraq and Syria.

- Jihadist crescent -

"Jordan is worried because there is a jihadist crescent now surrounding the kingdom. It is a very serious threat," said Rantawi.

In an attempt to counter the influence of Jordanian Al-Qaeda-affiliated jihadists fighting in Syria, Amman in April approved a tough anti-terrorism law and arrested scores on their return.

"The key thing to bear in mind is that while it can carry out terrorist attacks in Jordan, there are too many constraints for the group (ISIL) to act in Jordan as it has in Syria and Iraq," Stratfor said.

Britain can't monitor all Syria fighters: ex-security chief
London (AFP) June 23, 2014 - British security forces will find it impossible to monitor all the estimated 500 jihadists fighting in Syria and Iraq after they return home, a former top intelligence official said Monday.

Richard Barrett, ex-head of counter-terrorism at the MI6 overseas security agency, told the BBC that authorities would have to try to identify the biggest threats.

Barrett said the number who had gone to Syria "could be as high as 500 by now".

His comments came after several young British men featured in a YouTube recruitment video for the jihadist group Islamic State of Iraq and the Levant (ISIL).

"The trouble is, you don't know which ones are coming back just wanting to get on with their lives and which ones are coming back quite severely radicalised," he said.

He said it would be an "enormous challenge" and added that there was "absolutely no way" the security services could follow all of them, "that's out of the question."

"Clearly they'll have to prioritise and they'll have to choose those that they think are likely to pose the greatest risk," he added.

"Beyond that I think they'll have to rely very much on members of the community and other people expressing their concern and worry about the behaviour of perhaps their returned friend or family member."

ISIL began by fighting against the Syrian regime of Bashar al-Assad, but it is now leading Sunni militants making major advances in neighbouring Iraq.

The father of two young British fighters -- Nasser Muthana, 20, who appeared in the ISIL video, and Aseel Muthanam 17 -- said they had "betrayed" Britain.

"This is my country. I came here aged 13 from Aden when I was orphaned," father Ahmed Muthana told the Guardian newspaper.

The Daily Mail newspaper said a mosque in their home town of Cardiff, where the brothers worshipped, had played host in 2012 to Saudi cleric Mohammed al-Arifi, who has called for holy war and the overthrow of the Assad regime.

The third Briton appearing in the video was on Monday identified as Abdul Raqib Amin from Aberdeen in Scotland, according to British media reports.