RIYADH (Reuters) - Saudi Interior Minister Mohammed bin Nayef, perhaps the most powerful younger prince in the ruling al-Saud family, is shaping Riyadh's new emphasis on protecting the kingdom from a fresh wave of Islamist militancy inspired by the war in Syria.

The United States pulled out the stops for him when he visited Washington last week to prepare for President Barack Obama's fence-mending trip to Riyadh next month.

Secretary of State John Kerry, National Security Adviser Susan Rice, Central Intelligence Agency chief John Brennan, Homeland Security Secretary Jeh Johnson, Federal Bureau of Investigation director James Comey and National Security Agency director Keith Alexander all sat down with the 54-year-old, a veteran of Saudi Arabia's fight against al Qaeda.

Prince Mohammed seems likely to be a central figure in the world's top oil exporter for decades to come. Many Saudis say he is a strong candidate to become king one day.

"He's now playing not only the role of Interior Minister, but also that of a senior diplomat and adviser to the king," said Robert Jordan, U.S. ambassador to Riyadh from 2001-03.

"He is probably someone destined for even greater responsibilities in Saudi Arabia in due course," he said.

In recent weeks Prince Mohammed's influence has become more apparent in Saudi policy on Syria, where the kingdom's rulers fear their support for rebels battling President Bashar al-Assad may inadvertently revitalize radicalism at home.

Officials in Riyadh have noticed an increase both in Saudi jihadis leaving for Syria and in online chatter supporting Islamist militancy. This month King Abdullah decreed prison terms of 3-20 years on Saudis who go abroad to fight.

Saudi Arabia's goal of toppling Assad, an ally of its regional rival Iran, is unchanged, but it is refocusing on countering militancy, and is trying to ease differences on Syria with Washington, say Saudi and diplomatic sources in the Gulf.

So Prince Mohammed, who has built trust with U.S. security officials over a decade of cooperation against al Qaeda in Saudi Arabia and Yemen, is playing a bigger role.

"He has a lot of credibility," said a diplomatic source in the Gulf. "The fact that he is going to Washington now to say terrorism is a shared concern is a smart message."

Obama's visit to Riyadh is aimed at smoothing three years of tensions between the two historic allies over policy on Syria, Iran and Egypt. Saudi Arabia still hopes to convince the United States to adopt a more muscular approach on Syria that would counter both Assad and the rebel groups closest to al Qaeda.

As the man behind Saudi efforts to crush an al Qaeda uprising last decade waged by veterans of wars in Afghanistan and Iraq, Prince Mohammed is acutely aware of the dangers posed by Saudi militants who have fought overseas.

That campaign was so successful that al Qaeda in the Arabian Peninsula sent an assassin to kill the prince in 2009, posing as a would-be defector before detonating a bomb hidden in his clothes.

Prince Mohammed narrowly survived that attack and was named interior minister in November 2012, inheriting a role held by his hardline father, the late Crown Prince Nayef, for 37 years.

GENERATIONAL CHANGE

The al-Saud family is now slowly transferring power to a younger generation, although it will probably be a decade or so before the interior minister or one of his cousins becomes king.

Unlike in European monarchies, the Saudi succession does not move from father to eldest son, but down a line of sons of the state's founder, King Abdulaziz, of whom only a few remain.

In a family that prizes experience and seniority, Prince Mohammed is already well placed, running a ministry that gives him a say in most big government issues and puts him in charge of the regional governors, who are all princes themselves.

It will be his generation that will have to tackle major challenges looming for Saudi Arabia, such as overhauling an unsustainable economy built on state handouts while managing social and political reform in a deeply conservative society.

While his views on such issues may prove pivotal to the kingdom's future, Prince Mohammed keeps them close to his chest, according to people who have spoken with him.

"Everybody thinks he is a good friend. Both liberals and conservatives will say 'I sat with Mohammed bin Nayef'. He cannot be labeled as one thing or another," said Jamal Khashoggi, head of a TV news channel owned by a Saudi prince.

One clue to his approach lies in a dingy suburb of eastern Riyadh, behind high sandstone walls topped with razor wire: the Prince Mohammed bin Nayef Centre of Counseling and Care.

Better known abroad as Saudi Arabia's controversial rehabilitation program for militants, the center is the only institution named for Prince Mohammed in a country where senior royals wear their attachment to favored causes with pride.

It blends Western techniques such as psychiatry with lectures by clerics from the kingdom's official Wahhabi school of Sunni Islam, who do not dispute the virtues of waging jihad, but who insist it is the prerogative of the state.

"He understands the connection between education, faulty religious teaching and extremism," said Jordan, the former ambassador, who said he spent many nights sitting up with Prince Mohammed discussing such issues after the September 11, 2001, attacks on the United States, in which 15 Saudis took part.

"His approach was to counter the source of those fraudulent and erroneous interpretations and to explain, using Koranic principles, the way extremists were twisting things," he said.

By showing mercy to militants, he was also working to show Saudis that the government was not the dictatorship al Qaeda had painted it as. The program has not been an undiluted success, with some of its alumni fleeing to Yemen to resume the struggle.

SECURITY APPROACH

For all his close connections with the West, liberal activists see Prince Mohammed's security-focused approach as a danger to civil liberties, and point to his ministry's record of locking up dissidents for criticizing the ruling family.

Since he became interior minister 15 months ago, several prominent dissidents have been jailed.

His critics see him less as the Western-educated fan of Hollywood action films described by foreign officials, and more as the heir to a steely security apparatus built up by his conservative father Prince Nayef, who was interior minister from 1975 until his sudden death in 2012.

"He has been in the role of dealing with terrorism violence for almost all his political life. He doesn't strike me as someone who would be able to listen to a reform agenda," said Madawi al-Rasheed, a critic of the al-Saud family and author of A History of Saudi Arabia.

But Mustafa Alani, a Gulf-based security analyst who has worked closely with the Saudi Interior Ministry and knows Prince Mohammed, disputed this view, describing him as "one of those who supports reform: ready to listen, and who thinks that people have the right to express their opinion".

However, as interior minister, the prince was bound to uphold Saudi laws banning protests or overt criticism of senior royals and clerics, Alani said, adding: "You're talking about the most sensitive job in Saudi Arabia. It controls everything from criminal issues to political security. To manage this and still enjoy a degree of popularity and trust is not easy."

(Reporting by Angus McDowall; Editing by Alistair Lyon)