The United Nations is planning for a possible increase in violence in Afghanistan when US troops withdraw after two decades, the global body's refugee chief said.

Filippo Grandi told AFP in an interview that he understood that international military operations like the one in Afghanistan "cannot be sustained forever".

But, he warned, "the withdrawal of American troops from Afghanistan and other troops as well is another indicator that violence may rise after that", he said. "We are making plans for it."

The Taliban have made huge gains across Afghanistan as the United States prepares to pull out the last of its troops from the country by September after 20 years of war -- even as peace talks between the Afghan government and the Islamist group have stalled. 

Many Afghans -- especially women, who have been largely shut out of peace talks between the insurgents and Kabul -- have long feared a return to the Taliban's repressive Islamist regime if the US withdraws.

Analysts also fear a descent into civil war if Kabul is left to face the Taliban alone.

The situation is already dire, with some 2.6 million Afghans living abroad as refugees at the end of 2020, according to latest UN figures.

- Crisis 'can start again' -
Similar concerns have been raised about French President Emanuel Macron's recent announcement it will wind down its 5,100-strong Barkhane force, which has battled jihadists in the Sahel for the past eight years.

Macron said he instead sees France's future presence as being part of the so-called Takuba international task force in the Sahel, in which "hundreds" of French soldiers would form the "backbone".

The plan, which means the closure of French bases, has fuelled fears that certain areas of the Sahel, in particular northern Mali, will pass completely into the hands of jihadist groups, as local authorities appear unable to restore their grip on the region.

Grandi stressed that such international military interventions could not be expected to carry on forever to ensure stability.

But he said it was vital to make smart development and humanitarian investments while the troops are still on the ground.

"The problem is that if, while these troops are present, we don’t make the right investments in all aspects of the crisis, when the troops are inevitably pulled back, those symptoms will remain," he said.

"The crisis," he warned, "can start again, which means that after a few years, more troops have to be sent back." 

"This is our concern in Afghanistan for sure," adding that "in the Sahel the situation is still more complex and fluid".

He pointed to a fresh UNHCR report published Friday showing that some 750,000 people were forced to flee their homes in the Sahel last year alone.

"It is a negatively dynamic crisis," he said, urging the international community to take a "strategic approach that combines everything, humanitarian, development, investment in education, in economic prosperity".

That way, he said, "the security piece can proceed in a more coordinated manner".