BERLIN, May 6 (Reuters) -

French President Emmanuel Macron has warned that war could ensue if U.S. President Donald Trump withdraws from the 2015 deal in which Iran agreed to curb its nuclear program in exchange for sanctions relief.

"We would open the Pandora's box. There could be war," Macron told German weekly magazine Der Spiegel. But he added: "I don't think that Donald Trump wants war."

Trump is set to decide by May 12 whether to pull out of the Iran deal. Trump has all but decided to withdraw but exactly how he will do so remains unclear, two White House officials and a source familiar with the administration's internal debate said on May 2.

Trump could still figure out a way to stay in the deal between the Islamic Republic and six world powers -- Britain, China, France, Germany, Russia and the United States.

Macron urged Trump not to withdraw when he met the president in Washington late last month.

Britain, France and Germany remain committed to the accord but, in an effort to keep Washington in it, want to open talks on Iran's ballistic missile program, its nuclear activities beyond 2025 - when key provisions of the deal expire - and its role in Middle East crises such as Syria and Yemen.