Saudi authorities have uncovered a network of Islamic extremists, arms and sophisticated equipment operating in sleeper cells all over the kingdom.

Saudi officials say they only became aware of the terrorist cells during a crackdown with more than 15 raids following the May 12 suicide bombings in the Saudi capital.

In the latest gunbattle with extremists, security forces checking an illegally parked truck were fired on Tuesday in Riyadh (search), the second time police were attacked by militants in as many days.

A day earlier, Saudi police arrested 10 militants who allegedly belonged to a terrorist cell planning to attack a British target.

British Airways (search) suspended flights to the kingdom Wednesday after Britain's Department for Transport said it received "credible intelligence of a serious threat" to British aviation interests.

Adel al-Jubeir (search), a Saudi foreign policy adviser, said he was confident the terrorists would not have been successful because of increased security at airports throughout the kingdom. "Our airports in Saudi Arabia are very secure and we are determined to keep them that way," al-Jubeir said Wednesday in a televised interview.

Nonetheless, the State Department on Wednesday updated its travel alert on Saudi Arabia, citing the potential for further terrorist attacks on civilians there and urging Americans to exercise extra caution in the Persian Gulf region.

Last month, police found underground arsenals at farms in central and eastern Saudi Arabia, and cars and trucks ready for use as bombs. They have also discovered dozens of fake passports, surveillance equipment and donation boxes.

"The extremists had infiltrated and developed sleeper cells in Saudi Arabia to an extent that neither society nor the authorities were aware of. I believe they were still in the process of getting organized and setting themselves up when they were first raided by police" in May, said Mishari al-Thaidi, an expert on militants and a journalist with the pan-Arab Asharq al-Awsat newspaper.

The raids have foiled many plots, some on an even larger scale than the Riyadh bombings, which killed 26 people and nine attackers, according to Interior Minister Prince Nayef. The May 12 bombings targeted Riyadh housing compounds where foreigners as well as Saudis live; among the Saudi dead was Mohammed al-Balheed, son of the deputy governor of Riyadh.

Since the Riyadh bombings, Saudi authorities have arrested more than 200 suspects. More than a dozen militants and at least 10 security men have also died in gunbattles with extremists.

The extent and sophistication of the arsenal that has been uncovered and the presence of militant networks in Saudi Arabia's major cities indicates how powerful and far-reaching they are, al-Thaidi said.

"It's clear that they have sympathizers all over the Muslim world, including many young Saudis vulnerable to the call for jihad (holy war), more recently because of the U.S. war in Afghanistan and Iraq. The portrayal of those as crusader wars against Muslims makes it easier for Al Qaeda to gather recruits," al-Thaidi said.

Sulaiman al-Hattlan, a research associate at Harvard University's Center for Middle Eastern Studies, says the extremists' strength is the result of years of mobilizing, organizing and raising money on the pretext of helping embattled Muslims in Afghanistan, Bosnia and Chechnya.

"Al Qaeda took advantage of people's sympathy with Islamic causes. No Saudi citizen would have contributed money thinking it would be used against his own society. But now they have discovered that not all of the money went to help the needy in Afghanistan and is perhaps coming back to finance extremists inside Saudi Arabia, and they feel betrayed," al-Hattlan said.

Journalist Mansour Al-Nogaidan says the extremists have been forming cells and growing in strength for years, right under the nose of the authorities, who looked the other way or believed Saudi Arabia would not be targeted.

"Even before Sept. 11, people inside Saudi Arabia were talking openly about jihad. But the authorities were in denial and did not deal seriously with the problem. They thought they would be safe if the attacks were happening overseas. They allowed anti-American and anti-Western discourse and did not believe it would turn against them," Al-Nogaidan said.

"It's only when they started acts of violence inside the kingdom that the authorities started dealing seriously with them. But the kind of extremist thought nurtured here does not die in a day or two, and comes back to haunt you in the end."