Despite an increasingly sophisticated arsenal, Iran's medium- and short-range ballistic missiles have major shortcomings that make them particularly vulnerable to pre-emptive strikes.
The majority are liquid-fuel propelled, requiring a time-consuming missile launch, according to reports.
Iran also has to rely to some extent on others, particularly North Korea, for certain key missile components and materials in its medium- and short-range missile programme.
Export controls and sanctions have made it increasingly difficult for Tehran to acquire the best of such items.
The United States on March 30 announced sanctions targeting several entities it says are involved in procuring supplies for Tehran's nuclear programme.
The move "reinforces" Washington's commitment to prevent Iran from developing ballistic missiles, said Treasury Department under secretary for terrorism Brian Nelson in a statement.
He said that while the United States continues to seek "Iran's full return" to the 2015 nuclear deal, it "will not hesitate to target those who support Iran's ballistic missile programme" with more sanctions.
"We will also work with other partners in the region to hold Iran accountable for its actions, including gross violations of the sovereignty of its neighbours," he said.
The sanctions target Iranian national Mohammad Ali Hosseini and his "network of companies" as suppliers of the programme, according to the statement.
They follow "Iran's missile attack on Erbil, Iraq on March 13 and the Iranian-enabled Houthi missile attack against a Saudi Aramco facility on March 25, as well as other missile attacks by Iranian proxies against Saudi Arabia and the United Arab Emirates", it said.
Iran's missile programme has undergone some improvements in the past two decades -- including extending the range and accuracy of its missiles, developing cruise missiles, and building mobile launchers and underground silos.
The majority of Iran's heavy artillery rockets and ballistic missiles are tactical or short-range, which means they have a range of less than 500km, according to a Congressional Research Service study published in January 2020.
Most of Iran's ballistic missiles are liquid-fuelled Scud-B and Scud-C derivatives, known in Iran as the Shahab-1 and Shahab 2, respectively, with a majority likely being Scud-Cs, it noted.
"Iran has less than 100 SRBM [short-range ballistic missile] reusable transporter-erector-launchers (TELs) and likely has hundreds of SRBMs," the report said.
Meanwhile, Iran has perhaps fewer than 50 medium-range ballistic missile (MRBM) launchers and an unknown number of associated missiles, the report said, citing the US National Air and Space Intelligence Centre (NASIC) in 2017.
Iran's main MRBM is the Shahab-3, a liquid-fuelled ballistic missile imported from North Korea and based on the No-dong 1, and its variants.
However, the liquid fuel systems of the Shahabs require a time-consuming missile launch, the US Institute of Peace (USIP)'s Iran Primer noted in January 2021.
Such systems must be fuelled at a pre-determined launch site for two to four hours before being launched, making them vulnerable to pre-launch strikes by adversaries.
While both the Shahab-1 and Shahab-2 are road-mobile systems, wartime experiences have shown these missiles "tend to operate within a radius of about 100km or less from their bases because of the need to ensure operational security and to be able to maintain key logistics support", USIP said.
The destruction of short- and medium-range TELs pre-launch would be devastating for Iran, and would complicate Iranian attack plans.
The effectiveness of Iran's short- and medium-range ballistic missiles would be questionable even if they were to make it into the air.
"The military utility of Iran's liquid-fuel ballistic missiles is limited because of poor accuracy, so these missiles are not likely to be decisive if armed with conventional, chemical or biological warheads," according to USIP.
"The successful destruction of a fixed military target, for example, would probably require Iran to use a significant percentage of its liquid-fuel missile inventory," it added.
"Against large military targets, such as an airfield or seaport, Iran could conduct harassment attacks aimed at disrupting operations or damaging fuel-storage depots."
"But the missiles would probably be unable to shut down critical military activities. The number of transporter-erector-launchers available and the delays to reload them would also limit the impact of even a massive attack."
Tehran, however, could use its missiles as a political or psychological weapon to terrorise an adversary's cities and pressure its government, USIP said.
"Such attacks might trigger fear, but the casualties would probably be low -- probably fewer than a few hundred, even if Iran unleashed its entire ballistic missile arsenal and a majority succeeded in penetrating missile defences."