Secretary of State John F. Kerry defended the Obama administration’s carrot-and-stick approach to nuclear negotiations with Iran, saying Sunday that the conciliatory strategy needs to be given a chance to work — while vowing that the U.S. is prepared to use force if necessary to keep the Islamic republic from developing a nuclear bomb.

“We can’t let mythology and politics start to cloud reality,” said Mr. Kerry, who dismissed criticism that the administration has done a poor job leveraging American power in international talks — the latest round of which closed over the weekend without a breakthrough — over Iran’s disputed nuclear program.

“The president has been willing and made it clear that he is prepared to use force with respect to Iran’s weapon, and he has deployed the forces and the weapons necessary to achieve that goal if it has to be achieved,” Mr. Kerry said during an interview with NBC.

Congressional lawmakers, as well as U.S. allies including France and Israel, have expressed concerns that the Obama administration has veered dangerously close to making too many concessions in its pursuit of a deal for Iran to reconfigure its nuclear program and open it to close international scrutiny in exchange for lifting U.S.-led sanctions.

In order to lay the groundwork for such a deal, the Obama administration has spent the past month making rare diplomatic overtures toward Iranian President Hassan Rouhani, whom many Western analysts deem a moderate.

Despite the otherwise aggressive tenor of his remarks, Mr. Kerry defended the administration’s strategy of reaching out to Iran. “You have to act in some good faith, and an effort to be able to move towards the goal you want to achieve — if, as their act of good faith, they freeze their program and allow us absolutely unprecedented access to inspection and do other things,” he said.