US President Barack Obama walks through an honor cordon as the arrives for the 134th Commencement Exercises of the United States Coast Guard Academy in New London, Connecticut, May 20, 2015. [Photo/Agencies]

Obama pointed to severe drought in Nigeria as creating instability that was exploited by militant group Boko Haram, and said crop failures and high food prices in Syria fueled unrest that led to civil war.

"Around the world, climate change increases the risk of instability and conflict," he said. "Make no mistake, it will impact how our military defends our country. So we need to act, and we need to act now."

"Denying it or refusing to deal with it endangers our national security," he added. "It undermines the readiness of our forces."

Obama has been trying to build support for an international agreement to cut greenhouse gas emissions. Global talks on the deal are slated for Paris in December.

This summer, his administration will finalize regulations to cut carbon emissions from power plants, a step that has been opposed by Republicans who control Congress.

Likely Republican presidential contender Jeb Bush said of Obama's comments: "If the president thinks this is the gravest threat to our national security, it seems like he would say, 'Let's expand LNG (liquefied natural gas) as fast as we can to get it into the hands of higher carbon-intense economies like China and other places.'"