VIENNA — As they gathered last week, perhaps for the last time, diplomats from countries that brokered the Iran nuclear accord publicly and uniformly declared it was working — despite a death watch over the deal in Washington.

But behind the closed doors of the chandeliered room, complaints spilled forth about President Trump’s threats to tear up the agreement.

Iranian envoys said the warnings from Washington had scared away investors who had been expected to bolster the country’s economy after the deal was negotiated in 2015, according to participants. The American delegation countered that though the deal had effectively limited Tehran’s nuclear program, it had failed to curb Iran’s support for regional terrorism.

And when he emerged from the meeting, Brian H. Hook, the chief American representative to the talks, called the nuclear issue “important — but it is just one aspect of the threats from the Iranian regime.”