More than 350 U.S. veterans and national security officials have signed a letter calling on President Obama and members of Congress to support the Defense Department’s efforts to invest in alternative fuels.

“As a country, we must support efforts inside and outside the Department of Defense to reduce dependence on fossil fuels, deploy clean energy technology, and move our nation toward energy independence,” the letter says. “It is vital to our national security, our economic security, and our obligation to the brave men and women who serve in missions around the world.”

The letter comes amid a fight over amendments to Senate defense authorization legislation that limit the Defense Department’s ability to buy biofuels.