Some of Mali's top gold producers said a new law to allow the military-led government to increase its ownership of mines should not apply to existing operations, but analysts said it was likely to deter future investment.

In line with a rise in resource nationalism across the world, spurred on by strong commodity prices, the code adopted by Mali's Parliament - would allow the state and local investors to take stakes as high as 35% in mining projects compared with 20% now.

It will become effective once signed by President Assimi Goita, although it is unclear when that will be.

Mining companies operating in Mali said producing mines would be safeguarded by previous conventions, which could allow them to seek international arbitration if necessary.

The world's second largest gold miner Barrick said its CEO Mark Bristow has personally engaged with the current leadership over their proposed law.

"We are optimistic that, as in the past, we will find a mutually acceptable way to keep gold shining for Mali," a spokesperson told Reuters, saying Barrick has had "constructive relationship with successive governments".

Gold mining accounts for 9% of Mali's gross domestic product and half of state revenues.

Lassana Guindo, the ministry of mines' technical adviser told Reuters Mali wants to maintain its attractiveness, stability and respects commitments to investors, although he declined to say when the law would come into force and whether it would only impact new projects.

"We will have to wait for its promulgation and the implementing decrees, especially the implementing decrees. There are things that are still in progress," he said.

Until now gold mines, concentrated in the south of Mali and around the capital, away from the more unstable north of the country, have been largely shielded from instability and volatile tax regimes that have deterred investment in much of West Africa.

Vancouver-based B2Gold Corp which is expanding output at its flagship Fekola mine in Mali, said the operation is unlikely to be materially impacted by the law change during its lifespan.

"An existing mining project like Fekola Mine with a convention in compliance with the mining in place at the time the convention was entered into should not be impacted by the new mining code," B2Gold told Reuters.

Another operator Hummingbird Resources said investors would be monitoring developments.

"It would ultimately need to be factored into any future investment decision for potential projects that fall under the jurisdiction of this new mining law," a Hummingbird spokesperson told Reuters.

Analysts predicted investment would shrink as miners anticipated the new law could herald deeper change.

"It will likely discourage existing operators from developing new projects, deter foreign miners from investing in Mali," Mucahid Durmaz, senior Africa analyst at Verisk Maplecroft, said.

"There is a risk the new mining law is the thin end of the wedge. Miners will be concerned about the potential for further demands down the line, such as re-negotiating existing contracts."