Iran is covertly expanding an underground network of ballistic missile construction and testing sites, despite new U.S. sanctions aimed at deterring the Islamic Republic’s illicit program, which is believed to be focused on the delivery of a nuclear warhead.

Iran’s Revolutionary Guard Corps, or IRGC, has clandestinely moved to expand a network of front companies that primarily fuel the underground construction of ballistic missiles, according to sources following the country’s procurement system.

Iranian military leaders—who have recently dismissed new U.S. sanctions—are now focused on perfecting intercontinental ballistic missile technology, which would enable Iran to fire a nuclear-tipped warhead over great distances.

Iran maintains the "largest inventory of ballistic missiles in the Middle East," according to U.S. intelligence assessments. The country has recently been moving materials to underground sites via a complex network of IRGC-controlled companies, according to a brief on the procurement activity released by the Foundation For Defense of Democracies’ Center on Sanctions and Illicit Finance.

Tehran’s buildup continues despite United Nations resolutions calling Iran to halt ballistic missile activity. The issue has has emerged as a point of diplomatic conern in recent weeks as the United States attempts to convince nations such as Russia to help crack down on this behavior.

U.S. officials insist the administration will continue to press the Iranian regime on its program though specific details are not being made public.

"We will continue to take all appropriate multilateral and unilateral steps to address and counter threats from Iran's missile program," a State Department official told the Washington Free Beacon on Tuesday. "Beyond that, we're not going to telegraph any specific steps we might take."

As senior officials plan their response, Tehran is codifying an extensive range of companies and networks to feed its ballistic missile program, according to the FDD brief.

"Tehran has used commercial entities as fronts to procure sensitive technology or to provide goods for military purposes," according to the brief from resident Iran expert Saeed Ghasseminejad. "Today, a wide array of entities and sectors are thus likely involved in Iran’s ballistic missile program."

The program includes a network of IRGC-controlled suppliers that provide minerals and other raw materials required to fuel the missile program. This network has grown considerably over the years as the IRGC expands its grip on the Iranian economy.

"The ballistic-missile program requires expertise in both metallurgy and mining, and the IRGC has purchased a network of firms in both [sectors]," according to the brief. "Iran’s mining sector is a supplier of raw and processed materials for military uses, including ballistic missile production."

Iran controls "the world’s largest reserves of zinc and the second-largest reserves of copper." These precious minerals are used to reduce the visibility of missiles and "keep them from detection and interception," according to the brief.

The internal network feeds research at several underground facilities that continue to operate as the primary hub for Iran’s ballistic missile program.

"A ballistic missile program also requires the construction of testing and production facilities, above- and underground storage areas, and launch facilities," according to the brief. "In October 2015 and again in January 2016, the IRGC unveiled two underground sites for storing missiles. Given the sensitive nature of those programs, it is unlikely that the IRGC employed outside contractors to pursue them."

Experts speculate that these storage sites were built by the IRGC’s construction conglomerate, Khatam al-Anbiya, which employs some 5,000 contractors and hundreds of subsidiaries. The company also is developing several subway systems in Iran, which means it possesses the equipment needed to dig underground tunnels.

Additionally, Iran maintains multiple research centers that develop the technology needed to propel ballistic missiles over long distances.

The research centers study high-strength aluminum alloys and materials that can boost the strength of its rockets.

The IRGC, along with cooperation from Ministry of Defense and Armed Forces Logistics, "have expanded their industrial-financial empire" and now "control or indirectly influence many of the largest companies in Iran, especially in those sectors that provide dual-use goods and technology," the brief discloses.

Other materials used for this program, such as zirconium, are imported from countries such as China, Russia, South Africa, and a handful of other nations that work with Iran.

The U.S. intelligence community determined that "Iran’s ballistic missiles are inherently capable of delivering WMD," or weapons of mass destruction, based on its behavior.

This activity has expanded since the signing last year of the comprehensive nuclear agreement, the brief concludes.

There is little use for this program outside of the country’s nuclear weapon portfolio.

"Developing costly indigenous long-range ballistic missiles without acquiring nuclear warheads makes little sense, and Tehran’s insistence on developing ballistic missiles casts serious doubt on its intention to abstain from working on programs relevant to nuclear weapons after the JCPOA ends," the brief concludes.

The Obama administration argues that Iran’s ballistic missile tests violate U.N. resolutions governing the nuclear deal. However, nations such as Russia have defended Iran, claiming that the resolutions only suggest that Iran halt this activity.

Iranian military leaders say that new U.S. sanctions will not stop the program.

"Even if they build a wall around Iran, our missile program will not stop," Brig. Gen. Amir Ali Hajizadeh, commander of the IRGC aerospace arm, said Monday.

Ghasseminejad told the Free Beacon that the latest U.S. sanctions "are not enough."

"The sanctions are very limited, very narrow and ineffective," he said. "An effective sanctions regime against Iran’s ballistic missile program will target all the firms controlled by the IRGC, Iran’s defense ministry, and Iran’s supreme leader, all of who are in charge of the program."

Ghasseminejad said it would also include executives and others who work for these companies.

"These measures, if strictly enforced, can change Iran’s cost-benefit calculation and curb the progress of its ballistic missiles program," Ghasseminejad said.

"However, a permanent solution to the danger imposed by the Islamic Republic is to get rid of the regime. As long as the radical Islamists rule over Iran, they will use the country’s resource to support terrorism, acquire nuclear bombs and long-range ballistic missiles, and destabilize the region."