Climate change is threatening to force millions of people to become refugees and spark major wars that could "completely destabilise" the world, a leading general has warned.

And countries which attempted to deal with the coming crisis by resorting to "narrow nationalistic instincts" - for example, by building walls to keep out refugees - will only make the problem worse, according to Major General Munir Muniruzzaman, chairman of the Global Military Advisory Council On Climate Change (GMACCC).

He added that, while countries had talked a lot about the problems posed by global warming and how to address them, there did not seem to be "much action" on the ground.

The GMACCC was set up in 2009 to investigate the security implications of climate change and its members include serving and retired military officers from around the world, such as the UK's Rear Admiral Neil Morisetti and Brigadier General Stephen Cheney, a former US Marine.

Speaking ahead of the United Nations climate summit in Marrakesh next month, General Muniruzzaman said it was time to make good on the promises made at last year's historic meeting in Paris with global warming already contributing to flooding and droughts, threatening financial security and affecting people's health.

"In our analysis, we are seeing the risk is now becoming all-pervasive from climate change in the sense that it is touching multiple sectors … many of the sectors are being gravely challenged," he said.

"In some areas of the world, some of the issues we are touching on are becoming so severe they hold tremendous conflict potential."

He pointed to the recent diplomatic row between bitter regional rivals India and Pakistan, which both have large militaries and nuclear weapons, over water supplies.

"There was a possibility of a break down [of diplomacy] … which could have led to the first major water conflict of the world," he said.

The events of the Arab Spring and the Syrian civil war were also connected to unrest caused by droughts and crop failures.

General Muniruzzaman pointed to projections that sea-level rise could result in the loss of 20 per cent of Bangladesh's territory as early as 2050, which would force up to 30 million people to look for a new home.

"Imagine, with an international community unable to cope with a few thousand Syrian refugees, what will happen when millions of people are on the move," he said.

The reaction to the refugee crisis by some European countries - such as Hungary, Norway and the UK - has been to build walls and fences to keep people out.

But General Muniruzzaman said: "I'm very strongly of the opinion that walls are never a solution. You cannot build walls to stop people when they want to go to safety.

"If you build walls and high fences, they will break them and cross over. The risk people are taking when they cross the water [the Mediterranean] … many have drowned."

Instead of trying to hold back the tide of climate refugees, General Muniruzzaman said it would be better for the world to work out "international understanding and mechanisms" to enable mass movements of people to take place peacefully.

But the solution might need a significant rethink of the whole concept of the modern nation, which is said by some historians to have been born out of the Peace of Westphalia treaty in 1648.

"People have moved before. Environmental changes have forced people to relocate themselves historically," he said.

"What has become more difficult now is we have boxed ourselves into the Westphalian system of states.

"That is in conflict with nature, with the movement of people … we need to find a common ground.

"We need leaders with vision … we have to have a global solution to the problem, this is a civilisational problem.

"If we want to solve [these problems] with narrow nationalistic instincts, we will be adding more problems, not solving them."

Major wars and mass migration had the potential to "completely destabilise" not just countries and regions, but the entire world, the general said.

But he warned that most countries and most armed forces were "ill-prepared to meet the challenge for which they will be called upon to meet somebody and not too long away".

"We have to understand and meet the challenge, so we are not completely overwhelmed when they happen on the ground," General Muniruzzaman said.

"We don't have the proper strategies and policies in place to meet the security impacts of climate change.

"I would like to warn everybody we are way behind schedule to trying to find a solution to the problems we can see. In most cases we have been shying away from the problems we can absolutely identify and see.

"For a long time, we have been talking about the issues, but on the ground we don't see much action. As a soldier, I have a more action-orientated approach."