By Timothy Gardner

NORFOLK, Va. (Reuters) - Climate change poses a global security threat that all countries must fight together, a NATO general said this week, as U.S. President Donald Trump nears a decision on whether to pull out of the Paris climate deal.

The comments were the strongest yet from the U.S.-European military alliance about the importance of upholding the Paris accord. They come amid lingering tensions between the leadership of the North Atlantic Treaty Organization and Trump, who at one point called NATO "obsolete".

"There is a huge necessity that the U.N. continues to involve all nations and coordinate the action of all nations," to fight climate change, General Denis Mercier, NATO's supreme allied commander for transformation, told Reuters on the sidelines of a conference in Norfolk, Virginia.

"If one nation, especially the biggest nation ... if they do not recognize a problem, then we will have trouble dealing with the causes," of climate change, said Mercier.

Though he did not single out any country by name, the United States is the world's biggest economy and the second largest emitter of greenhouse gases after China.

Trump, a Republican, is weighing whether the United States should stay in the Paris climate agreement, a deal struck by nearly 200 countries in 2015 to limit global warming by reducing greenhouse gas emissions, and to provide funds to poor countries dealing with the effects of climate change.

Trump has complained that the United States was treated unfairly in the agreement because it would pay more than other countries to fight global warming. He is expected to decide in the next week or two on whether the country will leave the pact, or stay in with reduced commitments.

His Democratic predecessor, former President Barack Obama, had pledged a 26 percent to 28 percent cut in U.S. greenhouse gas emissions from 2005 levels, by 2025.

NATO's duty when it comes to climate change is to try to predict its impacts on geopolitical stability, said Mercier, who is French. He said risks include rising seas, water scarcity and the opening of access to resources in the Arctic - all of which he said are likely to bring about new conflicts that could involve the 28 NATO countries.

A global effort to stem climate change could help the world avoid some of these crises and conflicts, he said.

"It's not too late, but it is time," he added.

Norfolk, home to the world's largest naval base, already faces severe risks from rising sea levels. The main road to U.S. Naval Station Norfolk has experienced chronic flooding, and electric and water utilities supporting the base are threatened every time the waters rise.

"In some circles many public policy makers do not want to use (the words) global warming or climate change, but the reality is we are experiencing it, Norfolk Mayor Kenneth Alexander told reporters on the sidelines of the conference.

U.S. greenhouse gas emissions have fallen recently due to a switch from coal to natural gas and renewables in generating electricity. But many climate experts fear that if Washington leaves the Paris accord, or adjusts down its commitments, other countries could follow suit and global emissions could surge.

Many companies including ExxonMobil Corp, Microsoft Corp, and Arch Coal Inc, have urged the United States to stay in the Paris agreement, in part to retain their global competitiveness.