A collapse of the accord, known as the Joint Comprehensive Plan of Action (JCPOA), would send a “terrible signal” for other diplomatic efforts, Gabriel said on the sidelines of a UN meeting in New York on Thursday.
“What should motivate countries like North Korea or others to enter into negotiations in the future when the one example of such a deal is being destroyed?” he asked.
The remarks came two days after US President Donald Trump said the JCPOA was “an embarrassment” and “one of the worst and most one-sided transactions the United States has ever entered into.”
The Trump administration has twice verified Iranian compliance with the deal in notifications to the US Congress under an American law. But it has signaled that a third verification — due in mid-October — would not be forthcoming.
Gabriel said it was “tragic” that the nuclear agreement was being called into question.
“Now we will all try to convince the Americans in the remaining weeks ... that calling the agreement into question will not increase security,” he said.
The top German diplomat further noted that Iran was complying with the nuclear agreement.
On Wednesday, representatives from Iran and the six world powers highlighted the need for complying with the JCPOA amid US threats to dismantle it.
The JCPOA was reached in July 2015 and took effect in January 2016. Under the deal, Iran applied some limits to its nuclear program in exchange for the termination of all nuclear-related sanctions against it.
All the parties to the deal are determined to sustain it except Washington, which seems to be planning a unilateral withdrawal.
The Islamic Republic has repeatedly said that it would firmly and appropriately respond to any breach of the agreement.