As international negotiators work to meet a July 1 deadline for a deal designed to prevent Iran from getting a nuclear weapon, Iran's growing missile program threatens to derail the talks.

The Obama administration and its partners have agreed to keep discussions of ballistic missiles out of the nuclear talks after Tehran refused to continue them if that issue was not excluded. Administration officials insist, however, that they will press Iran on the issue outside of those discussions.

But concerns about the Middle East's largest and most diverse missile arsenal, and North Korea's role in helping Iran develop that arsenal, is a major concern in Congress and among the U.S. public, and could affect support for any nuclear deal. Those concerns focus on Iran's growing ability to use that arsenal to strike not just Israel, but Europe and eventually the United States — an ability that could quickly become critical if Tehran decides to break out of any nuclear deal.

"The missile program of Iran is something that never has even seemed to be seriously discussed throughout the course of these negotiations. And it's a very, very significant issue," said Rep. Ron DeSantis, R-Fla.

Though the administration has acknowledged the nexus between nuclear weapons and ballistic missiles' ability to deliver them, refusing to discuss Iran's program was a major concession that will represent "one of the biggest strikes against" any deal once it is submitted to Congress, he said.

"This agreement is something that I think is going to meet with a lot of stiff resistance in Congress, and one of the reasons should be Iran's missile program," DeSantis said. "Their missile arsenal undermines regional and international security."

Iran's ballistic missile arsenal, initially developed with North Korea's help, has been steadily expanding in size and quality, in spite of the U.N. Security Council's prohibition of the program. That cooperation has continued with the recent visit of North Korean experts to Iran, according to an Iranian exile group that has exposed clandestine elements of that country's nuclear program.

State Department officials say they are investigating the allegations, but would not give details.

"I don't have more to say on these specific allegations, which we are examining," spokesman Jeff Rathke said on May 28, refusing even to confirm or deny that the allegations have been raised in the nuclear talks.

Experts believe that Iran already has developed missiles capable of hitting Europe, and may have intercontinental ballistic missiles capable of hitting the United States by the end of the decade if the program continues to advance.

"Missiles are a cost-effective way for a country like Iran to pose an asymmetric threat to much more militarily sophisticated countries like the U.S. and are powerful weapons for coercion; therefore, Iran is motivated to keep and improve its arsenal," Rebecca Heinrichs, of the George C. Marshall Institute, told a House Foreign Affairs subcommittee in March.

"Iran wants more than a nuclear weapon. Iran wants to be able to credibly threaten its enemies with a nuclear missile," she said. "Any deal focused on Iran's nuclear program must include its missile program."

Missiles are not usually considered integral to efforts to prevent the proliferation of nuclear weapons. But the development of intermediate- or long-range ballistic missiles has proven to be an accurate litmus test of a country's nuclear intentions, as with Pakistan or North Korea, said Naval War College professor David Cooper.

"Time and again, real-world experience has demonstrated that the long-term time horizons, the vast expense and the international taboo of long-range ballistic missile programs ... really only make economic, political or military sense in the broader context of an ambition to become a nuclear weapons power," he said.

"This is arguably the most absolute indicator of whether a state's nuclear programs are peaceful, or are associated with nuclear weapons ambitions," he said. "Iran does say that their nuclear weapons programs are peaceful, but I would argue that the missiles may tell a different story."