Washington has said that it will issue no ultimatums on the window for a seventh round of talks on Iran's atomic activities but added a warning over potential "nuclear advances" in the meantime.
U.S. State Department spokesman Ned Price said that only Tehran can say when the talks -- begun in an effort to revive the 2015 nuclear agreement abandoned by Washington three years ago -- will continue.
"We're not imposing a deadline on these talks," Price said, "but...we are conscious that as time proceeds Iran's nuclear advances will have a bearing on our view of ultimately returning to the JCPOA."
President Joe Biden came to office in January pledging to pursue a return to the Joint Comprehensive Plan of Action (JCPOA), which traded an end to UN and other sanctions for checks on sensitive Iranian nuclear activities.
That deal was signed six years ago this month by Iran and the five permanent UN Security Council members (Britain, China, France, Russia, and the United States) plus Germany and the European Union but abandoned by U.S. President Donald Trump in 2018 and U.S. sanctions reimposed.
Six rounds of talks in Vienna have reportedly made progress but yielded no final deal.
The United States and European powers warned Iran last week that its latest uranium-enrichment efforts could imperil those negotiations.
Hours earlier, the International Atomic Energy Agency (IAEA) reported that Iran intended to enrich uranium to 20 percent -- a move that takes it a step closer to developing materials that could be used to make a nuclear weapon.
Tehran has consistently denied it is pursuing nuclear weapons but the IAEA and Western intelligence have accused it of lying and obfuscating in the past.