The first ever National Intelligence Estimate on Climate Change looks at the impact of climate on national security through to 2040.
Countries will argue over how to respond and the effects will be felt most in poorer countries, which are least able to adapt.
The report also warns of the risks if futuristic geo-engineering technologies are deployed by some countries acting alone.
The 27-page assessment is the collective view of all 18 US intelligence agencies. It is their first such look-ahead on what climate means for national security.
The report paints a picture of a world failing to co-operate, leading to dangerous competition and instability. It has been issued just ahead of President Joe Biden attending next month's COP26 climate summit in Glasgow, which is seeking international agreement.
It warns countries will try to defend their economies and seek advantage in developing new technology. Some nations may also resist the desire to act, with more than 20 countries relying on fossil fuels for greater than 50% of total export revenues.
"A decline in fossil fuel revenue would further strain Middle Eastern countries that are projected to face more intense climate effects," the report says.
Soon, it warns, the impact of climate change will be felt around the globe.
The US intelligence community identifies 11 countries and two regions where energy, food, water and health security are at particular risk. They tend to be poorer and less able to adapt, increasing the risks of instability and internal conflict. Heat waves and droughts could place pressure on services like electricity supply.
Five of the 11 countries are in South and East Asia - Afghanistan, Burma, India, Pakistan and North Korea - four countries are in Central America and the Caribbean - Guatemala, Haiti, Honduras and Nicaragua. Colombia and Iraq are the others. Central Africa and small states in the Pacific are also at risk.
Instability could spill out, particularly in the form of refugee flows, with a warning this could put pressure on the US southern border and create new humanitarian demands.