The long-stymied talks with Iran were re-energized after Hassan Rouhani, the new Iranian president, took office in August and declared that he wanted to resolve longstanding concerns about the country’s nuclear program so that punishing economic sanctions could be lifted.

But while the atmospherics in the talks have improved, administration officials are concerned that the window for negotiations might close if some way is not found to freeze Iran’s nuclear program. Some experts say it has already advanced to the point that Iran has the technological capability to make a bomb.

Olli Heinonen, the former deputy director general of the International Atomic Energy Agency, said in an interview that Iran had the technical know-how and sufficient quantities of uranium enriched to 20 percent purity to produce a crude nuclear explosive in two or three months if it decided to become a nuclear weapons state. It would take considerably longer for Iran to develop the means of delivering such an explosive by a ballistic missile, he added, but the nuclear threshold would have been crossed.

At the same time, American lawmakers have reacted to advances in Iran’s program by considering the imposition of even tougher economic sanctions, which Obama administration officials fear will harm the climate for negotiations.

“We’re looking for ways to put additional time on the clock,” said the administration official, who declined to be identified publicly under the administration’s diplomatic protocol for briefing reporters.

American officials would not provide details of the steps they wanted Iran to take to halt its nuclear program. But the administration official said such steps would cover the level of enrichment, Iran’s stockpiles of nuclear material and the capabilities of Iran’s nuclear facilities, as well as verification. Just how long the suspension would last also remains to be negotiated, but six months is one time frame that has been discussed.

“In response to a first step agreed to by Iran that halts their program from advancing further, we are prepared to offer limited, targeted and reversible sanctions relief,” the administration official said, without specifying what sanctions might be eased.