Higher global temperatures, melting polar ice and rising sea waters endanger millions of people, and the US armed forces must be ready for a new age when strife in volatile regions is multiplied by environmental catastrophe, they said.

Three former senior military officers testified to the Senate Foreign Relations committee on future national security challenges posed by global warming, as climate changes continued to carve a foothold in US politics.

"Climate change poses a serious threat to America's national security," said retired admiral and former US ambassador to China Joseph Prueher, one of eleven retired generals and admirals who compiled a recent report on the issue.

"Climate change will exacerbate many of the causes of instability that exist today -- those instabilities are part of the underpinnings of extremism."

Prueher said the US military should start planning now for the new era of environmentally spawned security threats, in the hope of mitigating some of the worst impact.

Retired air force General Charles Wald warned meanwhile of a set of serious security challenges posed by environmental change in Africa, an increasingly important source of oil exports to the United States.

An energy crisis could be sparked if rising sea-levels flooded the Niger delta or storms resulting from climate change damaged oil installations, he said.

"Millions of people could be displaced ... there really is not a controlled place in Nigeria for people to go," he said, warning of mass migration that could severely stress a population split between Christians and Muslims.

"These stresses would add dramatically to the existing confusion and desperation ... it makes the possibility of conflict very real," he said.

Wald cited political strife in Somalia as an example of how environmental crisis could spark social chaos -- in this case from alternating drought and floods which sparked mass migrations.

"Ungoverned spaces filled with desperate people are also the perfect recruiting grounds for terrorist groups," he said.

Retired vice admiral and former space shuttle astronaut Richard Truly said he dreaded the impact of global warming and any consequent rising sea levels on the Ganges Delta and India and Bangladesh.

"A small sea level rise literally measured in inches could displace millions of people ... as they turn around to walk to higher ground, there are also facing more of the most densely populated places on earth."

Many US security analysts already see South Asia as a vast geopolitical tinderbox.

Deltas of the world's other great rivers like Mekong, the Yangtze, the Nile and even the Mississippi could also suffer from rising water levels, he said, posing "potentially overwhelming security challenges."

Heavyweight senators also weighed in, reflecting the increasingly vocal climate change debate in US politics, following the release of former vice president Al Gore's movie "An Inconvenient Truth" and catastrophic Hurricane Katrina in 2005.

"Throughout human history, disruption on this scale almost always and everywhere meant war," said Democratic committee chair Senator Joseph Biden.

"We will either be drawn in too early, to mitigate the worst of the climate effects, or we will be drawn in later, when conflict has destabilized those countries."

The committee's top Republican, Senator Dick Lugar, said climate change dangers were allied to the threat to US security from excessive dependency on foreign oil imports.

Senator John Kerry pointedly remarked that he had tackled such issues in his failed 2004 Democratic presidential bid.

"Unfortunately they were hidden behind a cloak of completely false debate about the war on terror and a lot of misleading statements about what that war was all about," he said.

In the House of Representatives meanwhile an intelligence authorization bill about to be debated includes a requirement to set aside funds for a study of the national security threats posed by climate change.

The United States consumes around a quarter of the world's energy and causes nearly 30 percent of global greenhouse gas emissions.

Despite stressing his commitment to reduce emissions, US President George W. Bush has declined to adopt the so-called Kyoto protocol for countries to reduce them, arguing the cost would be prohibitive.

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