Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE on Monday said the U.S. may not be able fix the flaws in the Iran nuclear agreement in time to meet a White House deadline because the deal is contingent on getting three European countries to join efforts to monitor Iran's nuclear program.

When President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE waived sanctions against Iran earlier this month — a requirement under the Obama-era 2015 nuclear deal — he emphasized that he would not sign off on U.S. sanctions relief in May unless Europe agrees to help restrict Iranian missile testing and development as well as conduct inspections.

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Tillerson suggested that despite talks expected to get underway this week, he cannot force other countries to follow the U.S. timeline.

"The U.S. is under a bit of a timetable to deliver on what the president is looking for, but we don’t — we can’t set timetables for others," Tillerson told reporters accompanying him on a flight from London to Paris.

The secretary said he plans to address the matter by hosting for a working dinner Germany, France and Italy, countries the U.S. has had previous informal talks with about the deal, as well as the European Union.

"So this is just a continuation of the exchange of views on how we can address these flaws in the nuclear agreement, what kind of mechanisms could we use, but also how can we cooperate more on countering Iran’s activities that are not related to the nuclear program," he said, pointing to the U.S. concerns about Iran's "arms exports to Yemen and elsewhere."

"So we’re formalizing — I think maybe what we’re doing is we’re formalizing these groups to put a little bit more of structure around our working together to see what we can do together," he added.