"We're in a critical phase of the efforts to see whether we can revive the JCPOA," Malley told reporters on Monday during a press call, referring to the Joint Comprehensive Plan of Action and commonly known as the Iran nuclear deal, Xinhua reported. 

"We've had a hiatus of many months and the official reasons given by Iran for why we're in this hiatus are wearing very thin." 

Malley's comments followed his week-long trip to the Middle East and Europe over the Iran nuclear issue. US allies and partners shared "a deep and growing concern" about Iran's nuclear progress, he said. 

According to Malley, diplomacy is still a preferred path to address the Iran nuclear issue, but the United States and allies would also have to consider alternative tools. 

"We are always open to diplomatic arrangements with Iran, and we believe that this can only be resolved diplomatically. The question is whether the JCPOA can still be revived," he added. 

The US and Iranian officials began indirect talks in Vienna this April to restore the 2015 nuclear agreement. The two sides still have significant differences after six rounds of negotiations, which have been stalled since June. 

Malley stressed that the window for negotiations on a return to the JCPOA will not be open forever. "This is not a chronological clock. It's a technological clock. At some point, the JCPOA will have been so eroded because Iran would have made advances that cannot be reversed."