The head of the African Union is petitioning NATO nations to send forces to Mali to expel al-Qaeda-linked fighters, warning that convoys of Islamist extremists advancing on government-held towns represent a looming threat to the world.

Beninian President Thomas Boni Yayi, the chair of the African Union, injected new urgency into the Malian crisis while on a visit to Ottawa, by asserting that the threat exceeds the scope of a planned African force – and he presented a challenge to war-weary Western nations to join another dusty overseas war to dislodge jihadists.

It's a call that Prime Minister Stephen Harper rebuffed at a press conference with the African leader, insisting that his government "is not considering a direct Canadian military mission."

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While that left open some wiggle room for a limited contribution later, it ruled out so-called "boots on the ground," and Mr. Boni Yayi made clear that he is looking for more direct help from Western nations, including Canada.

The United Nations Security Council approved an international intervention in December.

The West African bloc ECOWAS is preparing a 3,300 strong force to bolster Mali's depleted army. The European Union is mustering 400 military trainers, but Mr. Boni Yayi said NATO should step in.

"When we speak of international forces, there is not just ECOWAS, not just the African Union, there are also other forces outside Africa to take into account the seriousness of the situation, and the required means," he said at a press conference with Mr. Harper. "In reality, NATO should join with our African forces – and I think our African forces will lead the way – as has been done in Afghanistan and in other places."

Mr. Harper is not the only NATO leader who will be reluctant. The United States and several other NATO countries are trying to wind down combat in Afghanistan next year. Though NATO took on a second foreign intervention in Libya in 2011, the prospect of a new counter-insurgency war in the vast, sparse desert and Sahel spaces of northern Mali to fight a mixed force of foreign extremists and local rebels is one they will approach with caution.

Until now, plans for direct military intervention focused on African troops. But Western nations are already concerned the small planned force of 3,300 will not be enough to bolster depleted Malian forces, which may field as few as 6,000 troops. Malian troops overthrew president Amadou Touré in March, sowing disarray – and al-Qaeda in the Islamic Maghreb, allied with local rebels, took control of northern Mali in April.

Now, advancing convoys of Islamist fighters moving toward government-controlled towns in central Mali have sparked new fears.

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The rebels were reported Tuesday to have advanced toward Malian army positions near the towns of Mopti and Sévaré in central Mali, but it was unclear whether it was a serious offensive or merely a bluff. Government forces said Tuesday they had repelled an attack by rebels advancing south toward Mopti.

That has raised concerns Islamist extremists will advance toward Mali's capital, Bamako, and destablize neighbouring countries. Mr. Boni Yayi said the crisis has now "exceeded" Africa's frame and has become a global problem. "Each day has a cost for the international community," he said.

Mr. Harper said "the development of essentially an entire terrorist region in the middle of Africa is of grave concern," but he said his government does not intend to send troops.

"The government of Canada is not considering a direct Canadian military mission," he said. "What we are doing, and will continue to do, obviously we're providing humanitarian assistance to this region, which is important. And we are consulting with and working with, and will continue, diplomatically, with our allies in the West and obviously with our friends in Africa, on ways that we can be of assistance."

Mr. Harper's rebuff, however, left open some wiggle room for Ottawa to offer some form of assistance, even if it rules out "boots on the ground" in Mali. Foreign governments have held informal discussions with Canadian officials about providing a small number of military trainers to assist the mission, and Western diplomats said Tuesday they still believe Canada will eventually deploy a small number – probably less than a dozen.

Some Western trainers are expected to help prepare forces from West African nations, possibly at a base outside Mali. Canadian officials, however, would not comment on what less "direct" military assistance Mr. Harper might consider.

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Robert Fowler, the former Canadian ambassador to the United Nations who was kidnapped by al-Qaeda radicals and held in northern Mali in late 2008 and early 2009, said he hopes Mr. Harper has not shut down all prospect of military assistance. African forces will need Western military capabilities, he said.

"I think allowing al-Qaeda to effectively have a homeland, to have an absolutely secure base from which they can plan and train and prepare and import foreign fighters, is not a good idea," he said.