A North Korean delegation that traveled to Iran in April for secret meetings about nuclear technology spent much of its time working with an Iranian agency responsible for weaponizing the country’s nuclear program, according to an exiled Iranian opposition group.

Reports Thursday said that at least three North Korean delegations traveled to Iran in late April to help advance the country’s nuclear weapons program. The National Council of Resistance of Iran (NCRI) released additional information claiming that the North Koreans primarily worked with an Iranian agency tasked with "manufacturing interior parts of nuclear warhead."

The NCRI’s claims sparked questions from reporters during the State Department’s daily briefing and forced the Obama administration to reveal it is looking into the reports.

Some has taken the NCRI claims as further proof of the extensive coordination between Tehran and Pyongyang on the nuclear front. In March, Iran was accused of housing illicit nuclear materials in North Korea to avoid detection by international inspectors.

The NCRI initially disclosed Thursday that based on sources inside Iran’s Revolutionary Guards Corps (IRGC), a seven-person delegation of North Korean defense ministry officials travelled to Iran at the end of April.

"The delegates included nuclear experts, nuclear warhead experts, and experts in various elements of ballistic missiles including guidance systems," the NCRI said, according to Reuters.

This was reported to be at least the third such meeting between high-level North Korean and Iranian officials. Another delegation is scheduled to be in Iran again in June, according to the NCRI, which in the past has revealed Iran’s clandestine nuclear enrichment facilities.

The NCRI further disclosed on Friday that the North Korean officials are working mainly with Iran’s Center for Research and Design of New Aerospace Technology, a regime-controlled organization primarily responsible for weaponizing Iran’s nuclear program.

The programs are run by senior IRGC officials, according to the NCRI, and the agency’s main "expertise is electronics area of research and manufacturing interior parts of nuclear warhead."

The opposition group also claimed the North Korean visits are being handled by Iran’s Ministry of Defense, which is also run by the IRGC.

Experts have warned for some time that Iran and North Korea are actively sharing nuclear technology and working to build up the Islamic Republic’s infrastructure.

If this coordination is in fact taking place, it could complicate efforts by international inspectors to catalog Iran’s nuclear program under any deal reached between Tehran and Western nations in negotiations.

The State Department said it is aware of the NCRI’s claims and that it is looking further into it.

"We have seen these claims, and we take any such reports seriously," State Department spokesman Jeff Rathke told reporters Thursday. "If I can perhaps anticipate one part of your follow-up, we’re examining the report but we don’t have any information at this time that would lead us to believe that these allegations impact our ongoing negotiations over Iran’s nuclear program."

Rathke described the claims as "serious" but said he is "not able to verify them" at this time.

"Any cooperation with Iran on proliferation-sensitive nuclear or ballistic activities would also violate relevant [United Nations] Security Council resolutions on Iran, including resolution 1929," he added.