"The United States remains deeply concerned about Iran's malign activities across the Middle East which undermine regional stability, security and prosperity," State Department spokeswoman Heather Nauert said in a statement, noting Iran's support for US-designated terrorist groups, militias and Syrian President Bashar al-Assad, as well as domestic human rights concerns.
Nauert also criticized Iran for continuing to develop its ballistic missile program, which the United States and other countries consider a violation of UN Security Council resolutions.
Late Monday, the Trump administration announced it planned to certify that Iran
was abiding by the terms -- but not the "spirit" -- of the Joint Comprehensive Plan of Action (JCPOA), an agreement reached in 2015 between Iran, the US and five other world powers under which Iran agreed to roll back its nuclear program in exchange for some sanctions relief.
That certification, which the administration is required to submit at regular intervals, effectively allows the deal to live on.
But President Donald Trump's decision to keep the deal alive -- at least for now -- appears to have been made reluctantly.
An administration official confirmed to CNN that the decision was temporarily put on hold Monday afternoon because the President wanted to consider his options and hear what would happen if he declared Iran non-compliant.
Bloomberg View first reported the last-minute delay.
The administration is currently reviewing its overall policy towards Iran, and Trump has indicated he may still pull the US out of the JCPOA at a later date.
In a statement, Iran's Foreign Ministry called the new sanctions "contemptible and worthless" and promised to retaliate "soon" with reciprocal sanctions against American interests.
Separately, Iranian government spokesman Mohammad Bagher Nobakht was quoted by Islamic Republic News Agency (IRNA) saying his country will not sit at a negotiation table with the United States as long as Americans "do not come to their senses," though it was not immediately clear whether he was referring to this latest set of sanctions or broader issues in the relationship.
Tillerson and his Iranian counterpart, Foreign Minister Mohammad Javad Zarif, have yet to speak directly to one another, according to Zarif.
On Monday, Zarif blasted US sanctions policy at a New York think tank event.
"I think people in Washington should get it through their minds that sanctions are a liability, not an asset," he told the audience at the Council on Foreign Relations.
Zarif also said a separate set of sanctions under consideration by the US Congress would be a violation of the JCPOA.
In total, the State and Treasury Departments have designated 18 entities and individuals under this latest round of sanctions for their support of Iran's military procurement for its Islamic Revolutionary Guard Corps -- a powerful government security agency tied to the country's hardliners.
Two of the entities were designated by the State Department "for engaging, or attempting to engage, in activities or transactions that have materially contributed to, or pose a risk of materially contributing to, the proliferation of weapons of mass destruction or their means of delivery," according to Nauert's statement.
The 16 entities and individuals sanctioned by the Treasury Department's Office of Foreign Assets Control included "an Iran-based transnational criminal organization and three associated persons," as well as companies and businessmen involved in military technology procurement, the Treasury Department said in a statement.