* Islamic State luring sub-Saharan Africans to Libya

* Spread of Salafism helps recruitment, analysts say

* Politicians demand checks on aid money from Gulf

By Emma Farge

ZIGUINCHOR, Senegal, March 30 (Reuters) - When Sadio Gassama decided to go into medicine, he started by giving free check-ups at his mosque in Senegal's poor southern region of Casamance. Now, the 25-year-old medical student says he is treating Islamic State fighters in Libya.

Until recently, many thought the peaceful, tolerant Sufi brotherhoods in countries such as Senegal could prevent more conservative and radical versions of Islam from taking hold in poorer parts of West Africa, like Mali and Niger.

But security experts say Gassama's story shows how the penetration of hardline Islamic Salafism, coupled with Gulf money and militant propaganda, is aiding recruitment, even from stable and democratic Senegal.

In particular, in their appeals to Africans, Islamic State propagandists are calling on doctors to make "hijrah", or pilgrimage, to their African stronghold of Sirte in Libya.

Pictures posted on Gassama's Facebook page before he joined Islamic State show him hugging his young niece. Now, he is brandishing a machine gun, his name stitched on to his military uniform.

Friends and family say Gassama's decision to join thousands of militants in Libya in December during the fifth year of his medical studies was sudden and unexpected.

His shocked father described him as a 'humanist' motivated by a desire to help others. A former professor called him a "brilliant student, incapable of hurting anyone".

But an interview with Gassama showed a darker side. Speaking from Sirte, he said he had been planning an attack in Dakar.

"Senegal is lucky. I was planning to commit an attack there in the name of the Islamic State before one of their contacts helped me go to Libya," he told Reuters last month via the internet. He could not be reached subsequently.

Friends said he took trucks to Libya via Mali and Niger, accompanied by another Senegalese man and paying his way with his student grant.

"I left Senegal a year after embracing the ideology of the Islamic State," Gassama said. "Joining ISIS in Libya was relatively easy and accessible. I wanted to contribute to the establishment of a caliphate in Libya."

Asked what he was doing there, he replied: "I am a jihadist doctor."

Islamic State propaganda and security sources confirm fighters from countries including Chad, Ghana, Senegal and Nigeria are already in Libya, where the group is consolidating its presence.

The number of sub-Saharan Africans is not known but they are thought to represent a minority of the 3,000-6,000 Islamic State fighters there, with most from North Africa and the Middle East.

However, there are concerns more will travel there along the same desert routes migrants use to reach Europe, as Gassama did.

"Libya is closer and easier to reach for some African fighters than Syria, and the political disarray there opens space for fighters to enter and operate," said Andrew Lebovich, a visiting fellow with the European Council on Foreign Relations who focuses on North Africa and the Sahel.

GULF DONATIONS

Across Africa's arid Sahel region, Western diplomats note an increase in conservatism, alongside tens of millions of dollars a year in charity donations from Gulf states.

In Niger, some religious leaders are calling for a "re-Islamisation" against the secularism imposed by former colonial power France.

This is already underway in the capital, Niamey, where some women wear the full veil and pay higher fares to avoid sharing taxis with men.

Gulf-financed bodies deny links to radical groups and say their money is for charity, but local sources say it can go astray.

"Contributions are intended for the poor and to build mosques but are often diverted in the wrong direction," said Bakary Sambe, director of the Timbuktu Institute and a coordinator for the Observatory on Religious Radicalism and Conflicts in Africa.

This foreign money and the migration of Senegal's youth to the cities has undermined the country's Mouride brotherhood, an old-established Islamic Sufi order which preaches tolerance.

In Gassama's home town of Ziguinchor, the mosque he attended in the HLM neighbourhood is funded by a Kuwaiti NGO called Africa Muslims Agency.

AMA director Almany Badji said it was one of more than 100 mosques it has financed in Casamance. The mosque Gassama attended at Dakar's Cheikh Anta Diop University also has Salafist leanings, Sambe said.

Gassama did not say who helped him join Islamic State more than a year ago, referring only to 'guidance' in Senegal.

"Through meetings with local scholars it became clear that jihad was my Muslim duty," he told Reuters.

His friends and family said the only change they noticed before he left was to a more Salafist dress code.

"His pants were shorter and did not reach all the way to the floor," said his father, Boucar Gassama, a retired civil servant, surrounded by Gassama's siblings in the shady courtyard of his house. "But I could not know he had been radicalised."

CALLS FOR REFORM

There is growing concern in West Africa about recruitment into Islamic State and other militant groups after attacks in Mali, Burkina Faso and Ivory Coast.

Modou Faye, Gassama's professor, says students need more guidance in reading the Koran, which is often rote-learnt at religious schools similar to one Gassama attended.

Mauritania has closed several Koranic schools for security reasons, officials said.

In Mali, where an Islamist insurgency is intensifying, some are calling for checks on mosques and NGOs.

"We must take stock of the potential risks of collusion between civil society and terrorists, better monitor places where radicalisation occurs, keep tabs on all suspect individuals like radical preachers and trace their funds," former Prime Minister Moussa Mara said.

But others say labelling peaceful Islamic groups as jihadists is risky. Depriving poor communities of services such as orphanages and free study trips to Saudi Arabia could provoke a backlash.

"A politician who attempts to regulate this risks losing his electorate," said Moulaye Hassane, researcher at the Institute of Research and Human Sciences and Niger's former ambassador to Saudi Arabia. "I think they are afraid." (Additional reporting by Abdoulaye Massalaki in Niamey and Diadie Ba in Dakar; editing by Giles Elgood)