Iran has pursued ballistic missile development even as international talks surrounding efforts to scale back its nuclear program have been ongoing, a confidential UN report said overnight Thursday-Friday.

“Analysis of Iran’s ballistic missile program remains a challenge,” the UN Panel of Experts report, obtained by Reuters, said.

“With the exception of several launches, periodic displays of hardware and one recent revelation of a new ballistic launch facility, the program is opaque and not subject to the same level of transparency that Iran’s nuclear activities are under IAEA safeguards,” the panel wrote.

“Among the most important items Iran is reportedly seeking are metals as well as components for guidance systems and fuel,” the panel report said. “Similarities between Iran’s ballistic missiles and space programs can make it difficult for states to distinguish the end-uses of procured items.”

In 2010, the UN passed a resolution that included a ban on Iran developing missiles capable of carrying a nuclear warhead.

An Iranian official, quoted by Reuters, said that the ballistic missile program would not be interrupted. “Iran purchases parts from various countries, including Russia and China, and then assembles missiles in Iran,” he said.

“Some Gulf countries have been involved in the missile delivery to Iran. Iran has never stopped its missile program and has no intention to do so; it gives Iran an upper hand.”

Earlier Thursday, negotiators from Iran and six world powers hunkered down to a second day of talks aimed towards what could be a historic deal on Tehran’s controversial nuclear program.

Tehran has often claimed that missiles should not be part of the ongoing talks with the P5+1 world powers.

Indications of how the talks were progressing in a rainy Vienna were thin on the ground, however. Both sides warned on arrival on Tuesday that the negotiations would be hard.

After three earlier rounds, this time Iran and the five permanent members of the UN Security Council plus Germany aim to start drafting the actual text of what could be a landmark agreement.

Success could help Tehran and Washington normalize relations 35 years after the Islamic Revolution toppled the autocratic US-backed shah, but failure could spark conflict and a regional nuclear arms race.

The parties want to get a deal by July 20, when a November 2013 interim deal — under which Iran froze certain activities in return for some sanctions relief — expires.

This could be extended, but time is of the essence with hardliners on both sides — members of the US Congress and arch-conservatives in Iran — skeptical of the process and impatient for progress.

The United States, Russia, China, Britain, France and Germany want Iran to radically scale back its nuclear activities in order to make any dash for the bomb virtually impossible and easily detectable.

In return, the Islamic Republic, which denies wanting atomic weapons, wants the lifting of all UN and Western sanctions, which have caused its economy major problems.

Even though there have been indications of some narrowing of positions, for example on the Arak reactor, both sides are sticking to the mantra that nothing is agreed until everything is agreed.

“Quite frankly, this is very, very difficult. I would caution people that, just because we will be drafting, it certainly doesn’t mean an agreement is imminent or that we are certain to eventually get to a resolution of these issues,” a senior US official said Tuesday.

The talks are tentatively scheduled to last until Friday, with Iranian Foreign Minister Mohammad Javad Zarif telling national media on Tuesday that he expected three more rounds before July 20.