Rob Malley said after a weeklong trip that both European and Gulf Arab nations backed a peaceful solution on ending Iran's nuclear program.

But he reiterated that the United States had "other options" -- a warning made by Secretary of State Antony Blinken earlier this month alongside his counterpart from Israel, which has threatened to attack.

"We will continue to pursue diplomacy even as we pursue other steps if we face a world in which we need to do that," Malley told reporters.

He said the Biden administration continued to believe that "this can only be resolved diplomatically."

"The window of diplomacy is never going to be closed," he said.

But on specifically renewing a 2015 nuclear deal, Malley said, "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," he said.

Iran was complying with a 2015 agreement reached with former president Barack Obama to scale down its nuclear work drastically -- until his successor Donald Trump withdrew the United States and reimposed sweeping sanctions.

Biden took office offering a revival of the deal, but six rounds of indirect talks with Iran have stalemated as Tehran pushes for a more widespread lifting of US sanctions beyond the measures issued by Trump.

"People sometimes criticize the US for talking about a Plan B. The Plan B that we're seeing implemented before our eyes right now seems to be the Iranian one which is to delay talks and accelerate their nuclear program," Malley said.

"We hope they choose that other path," he said, which would "give greater economic opportunity to Iran and a greater sense of security both to the region and to the world."