The United States slapped new economic sanctions against Iran on Tuesday over its ballistic missile programme and said Tehran's "malign activities" in the Middle East undercut any "positive contributions" coming from the 2015 Iran nuclear accord.

The measures signalled that the administration of President Donald Trump was seeking to put more pressure on Iran while keeping in place an agreement between Tehran and six world powers to curb its nuclear programme in return for lifting international oil and financial sanctions.

The U.S. government said it was targeting 18 entities and people for supporting what is said was "illicit Iranian actors or transnational criminal activity."

Those sanctioned had backed Iran's military or Iran's Islamic Revolutionary Guard Corps by developing drones and military equipment, producing and maintaining boats, and procuring electronic components, it said. Others had "orchestrated the theft of U.S. and Western software programs" sold to Iran's government, the Treasury Department said.

"The United States remains deeply concerned about Iran's malign activities across the Middle East which undermine regional stability, security, and prosperity," the State Department said in a statement.

It said the activities "undercut whatever 'positive contributions' to regional and international peace and security were intended to emerge" from the nuclear agreement.

On Monday, the Trump administration said Iran was complying with the nuclear agreement but it was also in default of the spirit of the accord and Washington would look for ways to strengthen it.

It was the second time Trump certified Iranian compliance with the agreement since he took office in January, despite having described it as "the worst deal ever" during his 2016 presidential campaign, criticizing then-President Barack Obama whose administration negotiated the accord.

"Even as we continue to work to prevent Iran from ever obtaining a nuclear weapon, we cannot look away while Iran threatens our country and our allies in ways beyond their nuclear threat," White House spokeswoman Sarah Sanders told reporters on Tuesday.

'POISON THE ATMOSPHERE'

The statement listed Iranian support for groups including Lebanon's Hezbollah and the Palestinian Hamas movement, the Syrian government of President Bashar al-Assad and Houthi rebels in Yemen.

Iran condemned Washington's sanctions announcement as "contemptible and worthless."

Iran "will reciprocate the move by imposing sanctions on a number of American natural and legal persons who have taken steps against the Iranian people and other Muslim nations in the region," the foreign ministry said in a statement.

Iranian Foreign Minister Mohammad Javad Zarif told CBS News the sanctions "poison the atmosphere" and violate the "spirit" of the nuclear agreement.

"We will look at it and see whether it violates the letter of the deal. And we will act accordingly," he said.

The Trump administration is reviewing policy on Iran, not only looking at Tehran's compliance with the nuclear deal but also its behaviour in the region which Washington says undermines U.S. interests in Syria, Iraq, Yemen and Lebanon.

Trump's reservations about the nuclear deal held up the White House's announcement on compliance, a U.S. official said. In the end, Trump agreed reluctantly to recertify the agreement after being advised repeatedly by his top national security aides to do so, another senior U.S. official said.

Behind the scenes, advisers argued that there was no alternative but to recertify the deal for now because the past sanctions regime the United States had with European allies against Iran is no longer in place and unilateral sanctions are not as effective as multilateral ones.

The State Department also called on the Iranian government to release U.S. citizens Baquer Namazi, Siamak Namazi, Xiyue Wang and other "unjustly detained U.S. citizens" and said it was deeply concerned about reports of their declining health.

"Iran should immediately release all of these U.S. citizens on humanitarian grounds," the State Department said.