(Foxnews/Reuters/AP) -- Speaking to reporters in Warsaw, Tillerson confirmed that Britain, France and Germany have all agreed to work on the 2015 agreement. Tilllerson ended his weeklong European trip in Poland, a trip intended to bolster the support needed to prevent the deal from collapsing.

Working groups, he said, have already begun to meet on efforts to agree on principles: "What is the scope of what we attempt to address and also how much we engage Iran on discussions to address these issues."

President Donald Trump has been outspoken on the issue, threatening to withdraw the United States from the deal unless the terms of the agreement are strengthened.

Earlier this month, Trump vowed to stop waiving U.S. sanctions unless the Europeans agreed to strengthen its terms by consenting to a side deal that would effectively eliminate provisions that allow Iran to gradually resume some advanced atomic work. Trump also wants tighter restrictions on Iran's ballistic missile program.

Iran has rejected renegotiation and has said that it has complied with all of the original terms of the deal, which was reached in 2015 under the Obama administration.

(Photo: U.S. Secretary of State Rex Tillerson speaks at the Warsaw Ghetto monument in Warsaw, Poland January 27, 2018. REUTERS)