U.S. Secretary of State John Kerry said he is optimistic a deal on Iran's nuclear program can be reached in coming days. Kerry was due in Switzerland Sunday for talks in what he said is the “last chance” for the U.S. to forge an interim deal to freeze Tehran's nuclear program.

Iran and six world powers -- the U.S., China, the U.K., France, Russia and Germany -- are facing a deadline at the end of March to agree on a framework deal for Iran to suspend its nuclear enrichment program. June 30 would mark the deadline for a final agreement.

“We believe very much that there’s not anything that’s going to change in April or May or June that suggests that at this time, a decision you can’t make now will be made then,” Kerry said, according to CBS News. “If it’s peaceful, let’s get it done. And my hope is that in the next days, that will be possible.”

Under the deal, Iran would curb its enrichment activities for 10 years in exchange for lifting U.N., U.S. and European sanctions on the country. Negotiations must still be hammered out on a number of details, including when the sanctions would be lifted and what kind of access international monitors would have to nuclear facilities. 

The talks are coming after 47 Republican lawmakers signed a letter to Iranian leaders warning of a deal. The letter said any agreement could expire after President Obama leaves office in 2016, a move that has produced a flurry of criticism for observers hoping for a deal.  

Kerry lambasted the letter in his comments Saturday. When asked if he would apologize for the letter during his meeting with Iran’s foreign minister, he responded: “Not on your life.”

“I’m not going to apologize for an unconstitutional, un-thought-out action by somebody who’s been in the United States Senate for 60-some days,” he said, according to CBS News.