The leaders of France, Germany and Italy say the Paris climate accord cannot be renegotiated as President Donald Trump has demanded.

French President Emmanuel Macron, German Chancellor Angela Merkel and Italian Premier Paolo Gentiloni said in a joint statement Thursday that they take note “with regret” the U.S. decision to pull out of the 2015 agreement.

The three leaders say they regard the accord as “a cornerstone in the cooperation between our countries, for effectively and timely tackling climate change.”

They added that the course charted by the accord is “irreversible and we firmly believe that the Paris Agreement cannot be renegotiated.”

Macron, Merkel and Gentiloni say they remain committed to the deal and will “step up efforts” to support the poorest and most threatened nations.