DAKAR: By seizing hundreds of hostages at a gas plant in the Algerian desert, Al Qaeda-linked militants angry at French intervention in Mali sent a clear message: they could strike anywhere in the Sahara.

Many experts now believe the sight of a former colonial power leading unprepared West African armies into war against Islamists in Mali could spark similar attacks across a swathe of smaller, more vulnerable nations to the south.

Islamist fighters who escape the French onslaught are likely to scatter, with some remaining in Mali to fight a guerilla-style war while others trickle across its porous borders into countries where pockets of radicalism already exist.

"This could lead to frustration amongst Muslims towards the French," said young Senegalese man Adama Sall, leaving afternoon prayers at a mosque in the Senegalese capital Dakar.

"In any intervention there is collateral damage, there are innocent people who could die. This could radicalise people."

Security experts have traditionally played down the threat of radical Islam across West Africa apart from Nigeria, where Boko Haram militants operate.

They cite the moderate form of Sufi Islam which predominates across the region, its largely open if ineffectual governments, and the limited number of past attacks by fundamentalist groups.

But the crisis in Mali has radically changed the dynamics in a region where a growing number of international firms operate, ranging from mining and petroleum to transport and construction.

Al Qaeda-linked groups which seized control of Mali's north had time to organise, recruit and rearm last year as regional governments wasted time trying to prise apart their alliance by offering talks to Ansar Dine, one of its main factions.

African nations now face the challenge of tracking groups of mobile Islamists across virtually meaningless borders while monitoring threats from radical Islam at home.

Paris had long pledged to back an African intervention in Mali but was determined to avoid French boots on the ground for fear of being painted as a crusading force. But images of French troops and armour in Mali are being beamed across the world, inflaming sentiment among some Muslims.

Aaron Zellin, a fellow at the Washington Institute for Near East Policy who monitors jihadi forums, said Mali had overtaken Syria as the top subject of online discussion.

"A lot of these people are more like online cheerleaders," he said. "But this could lead individuals to put away the keyboard and pick up an AK47 instead."

Islamists have repeatedly warned they will strike regional powers or Western interests if attacked. With the Algerian gas plant drama still unfolding, the group claiming responsibility threatened foreign companies with fresh attacks.

Experts warn that if the Islamists can hit Algerian interests, protected by security forces hardened by a bloody conflict against Islamists in the 1990s, West African nations with ill-equipped and inexperienced troops look vulnerable to a militant threat that has become increasingly international.

Among the militants killed in the Algeria hostage siege, Algerian security sources reported there were Egyptians, Tunisians, Libyans, a Frenchman and a Malian.

During the months they occupied northern Mali, West African security sources say Islamists in Mali lured a range of foreign recruits, including significant numbers from sub-Saharan Africa, previously under-represented within regional al Qaeda factions.

In Senegal, a neighbouring Muslim nation proud of its religious tolerance, Foreign Minister Mankeur Ndiaye warned this week that al Qaeda had set up sleeper cells in the country.

President Macky Sall called on all Senegalese to report suspicious activities by Muslims coming from abroad. "We must remain vigilant in our towns and villages as infiltrations exist," said Sall, who is sending 500 troops to Mali.

In Mauritania, bordering Mali to the west, both secular and Islamist parties across a deeply divided political spectrum have insisted the Islamic republic must stay out of Mali.

Before Islamists seized northern Mali, Mauritania was the country of the region most exposed to al Qaeda's activities. It has launched raids on Islamist camps across the border in Mali after attacks on its army and Western interests in the country.

"Mauritania will not get involved in the conflict," said Mohamed Yahya Ould Horma, vice president of the ruling UPR party. "We have already paid too high a price for acting alone against terrorist groups over the years."

Underlining the presence of radical thinking in a nation straddling Black and Arab Africa, a group of 30 Mauritanian religious scholars have called on Muslims across the region to protect jihadists from Mali.

"France wants to drive out extremists. But to where? Mauritania and Niger could be in trouble. Burkina Faso will face threats," said Kwesi Aning, an expert at the Ghana-based Kofi Annan International Peacekeeping Training Centre.

But it is Nigeria which could be the greatest concern. With the region's biggest oil reserves and economy, its security forces are already bogged down in on-off fighting with Boko Haram militants in the north.

Abuja has wavered between not wanting to overstretch its army by intervening in Mali and hoping the mission could stamp out links between homegrown and global militants.

By dispatching the first of 1,200 soldiers this week, President Goodluck Jonathan opted for the latter.

"They want to cut off the Islamist problem at the root," said Bismarck Rewane, CEO of Lagos-based consultancy Financial Derivatives. "Not only could it inflame things here, but in the long run it won't solve the problem of Boko Haram's insurgency, which is to do with inequality and poor governance."

Most West Africans, including the Malians themselves, have been largely supportive of French intervention while regretting the inaction of regional powers to come to Bamako's rescue.

However, the failure of democracy to improve daily life in some of the world's poorest countries has opened the door to Islamic organisations to play a bigger role.

Ultra-conservative Wahhabism, spread by preachers coming from the Gulf, has made inroads.

"This intervention (in Mali) makes the whole sub-region considerably more vulnerable," Aning added.

"We are going to see the spread of the fronts from Mali."