"They've been difficult to make a deal," Trump said of the Iranians before an audience of active-duty soldiers at Fort Bragg Army base in North Carolina after US officials said they were sending a second aircraft carrier to the Middle East.
"Sometimes you have to have fear. That's the only thing that really will get the situation taken care of."
During his address Trump also referenced the US bombing of Iran's nuclear sites in June.
Earlier, he said the deployment of the world's largest aircraft carrier was being made so "we'll have it ready" should negotiations with Iran fail.
He told reporters he thought talks with Iran would be successful but warned, "if they're not, it's going to be a bad day for Iran".
Oman facilitated talks between Iran and the US last week, which a spokesperson for Iran's foreign ministry said had allowed Tehran to gauge Washington's seriousness and showed enough consensus for diplomacy to continue.
The date and venue of the next round of US-Iran talks have yet to be announced.
Iran has said it is prepared to discuss curbs on its nuclear program in exchange for lifting sanctions but has ruled out discussions on its ballistic missile capabilities.
The US president travelled to Fort Bragg on Friday, local time, to meet special forces troops involved in the audacious January 3 operation to seize Venezuelan President Nicolas Maduro.
During the visit, Trump broke with presidential norms and delivered an openly political speech to the troops, blasting his political opponents and warning that Democrats would undermine the military if they won control of Congress in the midterm elections.
The US is sending the Gerald R Ford aircraft carrier to the Middle East, its newest and the world's largest carrier, which has been in the Caribbean with its escort ships and took part in the operations in Venezuela.
An official, who was speaking on condition of anonymity, said the carrier would take at least a week to reach the Middle East.
The Gerald R Ford will join the Abraham Lincoln carrier, several guided-missile destroyers, fighter jets and surveillance aircraft that have been moved to the Middle East in recent weeks.
US forces already have shot down an Iranian drone that approached the Lincoln on the same day last week that Iran tried to stop a US-flagged ship in the Strait of Hormuz.
Meanwhile, Iranians are beginning to hold 40-day mourning ceremonies for the thousands killed in Tehran's bloody crackdown on nationwide protests in January.
And Iran on Friday released on bail three prominent reformist figures arrested last week amid a sweeping government crackdown on dissent, one of their lawyers told the semi-official ISNA news agency.
Lawyer Hojjat Kermani said the freed activists included Azar Mansouri, head of the moderate Reform Front coalition, along with Javad Emam and Ebrahim Asgharzadeh.
He said he was not told what charges his clients may face. The releases come as authorities try to quell the country's bloodiest unrest since the 1979 Islamic Revolution through a campaign of mass arrests and intimidation.
The US-based rights group HRANA said about 53,000 people have so far been arrested, and the total number of killed had reached 7008, including 6509 protesters.
Reuters could not independently verify the figures, and Iranian officials could not immediately be reached for comment.
With AP
