Days after a letter from President Donald Trump reached Iran’s supreme leader, giving him a choice between negotiating a deal to end Iran’s nuclear program or U.S. military action to destroy it, the two sides remain far apart on the conditions that would allow such a conversation,

Iran has strongly rejected demands from the United States to halt its nuclear program, calling the latest pressure from Washington an act of “bullying.” The U.S. government recently sent a letter offering Iran two choices—either agree to negotiate a deal to shut down its nuclear activities or face military action.

China, Iran and Russia called for diplomacy over “pressure and threats” and an end to “all illegal unilateral sanctions” following talks on Tehran’s nuclear program Friday in Beijing, Chinese state media said.