WASHINGTON — US President Donald Trump said Saturday he will impose additional sanctions on Iran in an effort to prevent Tehran from obtaining nuclear weapons, adding that military action was still a possibility.

Trump, who was speaking to reporters at the White House, made his comments after recently calling off military action against Iran to retaliate for the downing of a US military drone.

"We are putting additional sanctions on Iran," Trump said. "In some cases we are going slowly, but in other cases we are moving rapidly."

The president said military action "is always on the table" against Iran.

But Trump also indicated he was open to reversing the escalation, adding he was willing to quickly reach a deal with Iran that he said would bolster the country's flagging economy.

"We will call it 'Let's make Iran great again.'"

Trump spoke to reporters as he prepared to depart Washington for the presidential retreat Camp David, where he said he would be deliberating on Iran.

Iran said on Saturday it would respond firmly to any US threat, the semi-official Tasnim news agency reported.

On Thursday, an Iranian missile destroyed a US Global Hawk surveillance drone. Tehran repeated on Saturday that the drone was shot down over its territory. Washington said the incident happened in international airspace.

A Pentagon spokeswoman, Commander Rebecca Rebarich, said on Saturday: "We stand by where we said the aircraft was operating in international airspace."

The US Federal Aviation Administration on Thursday issued an emergency order prohibiting US operators from flying in an oversea area of Tehran-controlled airspace over the Strait of Hormuz and Gulf of Oman. Some other international airlines are taking related precautions. — Agencies