By Angus McDowall

RIYADH (Reuters) - Tighter security in Saudi Arabia has made it hard for Islamic State to target the government so the militants are instead trying to incite a sectarian conflict via attacks on the Shi'ite Muslim minority, the Saudi Interior Ministry said.

Last week the Sunni group's leader Abu Bakr al-Baghdadi called for attacks against the Sunni rulers of Saudi Arabia, which has declared Islamic State a terrorist organization, joined international air strikes against it, and mobilized top clergy to denounce it.

He spoke after an attack on Shi'ite civilians, the first since 2006 by militant Saudis based inside the kingdom.

Islamic State has not claimed the shooting and the Saudis have not held the group responsible but they arrested more than 50 people including some who fought with Sunni jihadis in Syria or had been previously jailed for fighting with al Qaeda.

As the world's top oil exporter, birthplace of Islam and a champion of conservative Sunni doctrine, Saudi Arabia represents an important ally for Western countries battling Islamic State and a symbolic target for the militant group itself.

"Islamic State and al Qaeda are doing their best to carry out terrorist acts or crimes inside Saudi Arabia," Major General Mansour Turki, security spokesman for the Interior Ministry, told Reuters.

"They are trying to target the social fabric and trying to create a sectarian conflict inside the country."

The attack by gunmen in the Eastern Province district of al-Ahsa on November 3 killed eight members of the kingdom's Shi'ite minority who were marking their holy day of Ashoura.

Turki said he was not aware of any evidence that it was coordinated with Islamic State operatives outside Saudi Arabia.

He said improved government security, such as guards at possible targets, increased border defenses and surveillance, have made it much harder for militants elsewhere to organize violence inside Saudi Arabia such as al Qaeda's 2003-06 uprising which killed hundreds and led to the detention of more than 11,000 people.

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Although Saudi citizens have played important leadership roles in various al Qaeda organizations, Riyadh has not yet identified any in senior positions in Islamic State, Turki said.

However, the group tends to use Saudi members of Islamic State in its propaganda because of the kingdom's role as the leading Sunni state, he said.

"THEY WANT OUR PERSONALITY"

Riyadh is worried that the rise of militant Sunni groups, including al Qaeda affiliate Nusra Front and Islamic State, as participants in the Syrian war would radicalize Saudis who might then carry out a new wave of strikes inside the kingdom.

Although it has backed rebel groups fighting alongside jihadis against Syrian President Bashar al-Assad, Saudi Arabia has also taken steps to stop its people joining militants in Syria or Iraq or giving them money.

Turki said a royal decree in February imposing long prison terms for people who went abroad to fight or helped others to do so, and for people who gave moral or material support to militant groups had reduced the number of Saudi jihadis.

"One of the people we arrested (since the decree) was used by them (Islamic State) to write Friday sermons. Does this mean they do not have anybody capable of doing that? Of course not, but they want our language, our personality, to be reflected in their speeches," he said.

Since the decree was issued, the rate of Saudis traveling to Syria or Iraq for jihad had slowed sharply, while the rate of Saudis returning to the kingdom from those countries had accelerated, he said.

The authorities have identified between 2,000-2,100 Saudi citizens who have fought in Syria since its crisis began in 2011, of whom around 600 have returned, he said. Of those numbers, only about 200 had left Saudi Arabia since the February decree while around 170 had come back.

SECTARIAN ATTACK

The difficulty of getting its fighters past security and into Saudi Arabia has pushed Islamic State to try to incite sympathizers inside the kingdom to carry out their own attacks, Turki said.

Unlike the al Qaeda campaign last decade, the attack in al-Ahsa was not aimed at government, infrastructure or foreign targets, which are now better protected by security forces, but struck at unarmed Shi'ite villagers.

That showed the increasingly sectarian nature of jihadi ideology but also that tighter security had reduced the number of straightforward targets for militant attacks, Turki said.

The authorities detained 10 more people on Sunday for the attack, taking to 54 the total number of suspects arrested in 11 different Saudi cities.

"The situation is unlike 10 years ago when we had the first al Qaeda attacks. We were not ready at that time. Our public was not informed, our policemen were not trained or equipped for such a danger," he said.

(Editing by Anna Willard)