The French offensive in Mali will burden a defence budget that was expected to get relief from the withdrawal of French forces from Afghanistan, but the fighting might boost arms sales if the Rafale combat jet proves decisive in driving Islamist guerrillas out of the West African nation.

Asked how much the week-old operation would cost, French Defence Minister Jean-Yves Le Drian replied: "I cannot evaluate that yet. It will all be public knowledge of course."

He has already warned that it will take a long time to dislodge Islamist fighters who are highly mobile in the desert regions of northern Mali and who are careful to disseminate themselves among civilians in populated southern areas.

By comparison, the European Union has set a 15-month budget of 12.3 million euros ($16.5 million) for its share of the effort, which with 450 troops will be small compared with what France is likely to shoulder, and does not include salary or personnel costs.

The French defence ministry's external operations budget for 2013 is 630 million euros, about 90 million less than last year owing to anticipated savings from the Afghan withdrawal.

In 2011, France spent 1.2 billion euros to cover fighting in Libya, a different sort of operation based on an air campaign that lasted eight months and was closely coordinated with Britain.

On Monday, France said its overall budget deficit came to 87.2 billion euros in 2012, and it is trying to cut that back over the next two years.

The 2012 deficit was expected to come in at the government's target of 4.5 percent of economic output.

Paris has pledged to the EU to push the deficit down to 3.0 percent of GDP this year, with 37 billion in tax hikes and spending cuts to meet the target.

In Mali, help has come from Belgium, Britain, Canada, Denmark, Germany, and the United States, mainly in the form of transport planes to ferry troops and material from France and African countries that have agreed to contribute soldiers.

French C-160 aircraft are nearing the end of their expected lifespan and Harfang drones have been worn down by three years and 5,000 flight hours in Afghanistan.

"Military sources say the equipment being deployed will just about cover this kind of operation, but we are really pushing it," said Axel Poniatowski, a French deputy who presides the national assembly's defence committee.

The operation is also a test for the Rafale combat jet made by Dassault Aviation, a French group that is in talks with India for up to 189 aircraft, according to sources close to the discussions.

The possibility of an additional 63 jets being added to an expected order for 126 was raised during a visit by India's Foreign Minister Salman Khurshid to Paris last week, they said.

Indian media have estimated the value of the deal for 126 Rafales at $12 billion (nine billion euros).

The Rafale saw its first combat operations in Libya, but so far Dassault has not signed any binding foreign sales contracts.