A recent attempt to assassinate Saudi Arabia's security chief has raised concerns over new methods of hiding explosives, as well as Al Qaeda using Yemen to revive its operations against Saudi Arabia, experts say.

In August, a suicide bomber posing as a repentant militant blew himself up in the Jeddah office of security chief Prince Mohammed bin Nayef in the first known attack on a member of the royal Saudi family since Al Qaeda began a campaign in 2003.

Saudi media reports have suggested the bomber, who entered Saudi Arabia from Yemen in a prior arrangement with the prince, hid the device in his anus.

'This is indeed a new strategy ... to hide explosives inside the body, inside sensitive parts,' said Sheikh Mohammad Al-Najimi, professor at King Fahd Security College which trains thousands of police officers every year in counter-terrorism.

'I think they will try it again,' he said.

The explosion was probably activated by mobile phone, diplomats and experts say. It caused heavy damage to the room and even such small quantities could be lethal if used in a plane.

Regular airport procedures do not involve such body checks or use of sensors that would detect that kind of explosive.

'This is an international problem, not just for Saudi Arabia and the Gulf,' said Mustafa Alani, a security analyst in Dubai.

Diplomats said security at the diplomatic quarter in Riyadh and other sensitive points had been increased in the days after the attack but since then no visible changes could be seen.

Stability in the world's biggest oil exporting region is of global concern as Saudi Arabia, Kuwait, Qatar and the United Arab Emirates are key OPEC members and major dollar asset holders.

The incident confirmed Saudi fears that Yemen has become a new centre for Al Qaeda, where many Saudis on a list of 85 wanted suspect militants are thought to be. An insurgency in the north and separatist unrest in the south has created instability that al Qaeda can exploit, Saudi officials fear.

'There seems to have been no crackdown on suspects (inside Saudi Arabia) as some might have expected,' said a Western diplomat in Riyadh. An interior ministry spokesman told al-Riyadh on Wednesday that investigations were still ongoing.

Al Qaeda revival

Saudi Arabia managed with the help of foreign experts to halt a campaign of Al Qaeda in 2006 after a serious of attacks on foreign residential compounds, government targets and energy installations since 2003.

The Yemeni and Saudi branches of al Qaeda merged earlier this year to form the al Qaeda in the Arabian Peninsula (AQAP).

'Following the merger, Al Qaeda has prioritised attacks in Saudi Arabia and other Gulf states in an attempt to make itself regionally relevant,' said CTC Sentinel, a US-based publication that researches militant affairs.

Gulf states hope that a border fence Saudi is building can  end infiltration, especially from the over 1,500 km long, mountanous border with Yemen, the poorest country in the region.

In July, Saudi Arabia awarded European firm EADS a $2.3 billion deal to build a fence with command posts, cameras and sensors accompanied by coastal and air surveillance.

Analysts say that while it was a good idea, the Saudi fence project will not immediately end a tradition of smuggling everything from weapons to pilgrims through the porous border with Yemen. For one, it would take years to be ready.

'Al Qaeda will probably try to find a way around it,' said Theodore Karasik at the Institute for Near East & Gulf Military Analysis, adding that sea surveillance was also crucial.

CTC said Al Qaeda is trying to forge alliances with Yemeni tribes through marriages.

Diplomats say Crown Prince Sultan's absence from the country for almost a year due to illness has harmed security policy since he was a key figure organisating Yemeni policy.

'Saudi Arabia is helping Yemen much with intelligence but Sultan used to be the main contact for Yemeni tribes and other main players,' said another Western diplomat. – Reuters