By Mohammad Zargham and Steve Holland

WASHINGTON (Reuters) – The United States unveiled new economic sanctions against Iran over its ballistic missile program on Tuesday and said it was deeply concerned about Tehran’s “malign activities” in the Middle East.

The measures signaled that the administration of President Donald Trump was seeking to put more pressure on Iran while keeping in place for now a 2015 agreement between Tehran and six world powers to curb its nuclear program 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.

On Monday, the Trump administration said that 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 election campaign, criticizing then-President Barack Obama whose administration negotiated the accord.

“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.

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.

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

There was no immediate response from Iran to Tuesday’s sanctions announcement. On Saturday, Iran blamed Trump’s “arbitrary and conflicting policies” for global security threats.

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.

“He sometimes, when he has to do something that he doesn’t really want to do but he knows that he has to do, or it’s the least bad of all the alternatives, he can be very recalcitrant about it. He lets his opinion be known,” the senior U.S. official told Reuters.

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 multi-lateral ones.

“If we simply get out of the deal the Europeans will no longer be with us,” the senior official said.

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.

(Reporting by Mohammad Zargham and Steve Holland; additional reporting by Susan Heavey; Writing by Yara Bayoumy; Editing by Marguerita Choy and Grant McCool)