Trump said US ships, aircraft and personnel with additional ammunition and weaponry would remain in place to destroy, if necessary, "a substantially degraded enemy", but expressed confidence that a lasting deal would be agreed and followed.
"If for any reason it is not, which is highly unlikely, then the 'Shootin' Starts,' bigger, and better, and stronger than anyone has ever seen before," Trump said in a social media post, adding that contrary to "fake rhetoric", Iran had agreed not to pursue nuclear weapons and to reopen the Strait of Hormuz.
"In the meantime our great Military is Loading Up and Resting, looking forward, actually, to its next Conquest. AMERICA IS BACK!"
Though both the United States and Iran declared victory in a five-week-old war that has killed thousands, their core disputes remained unresolved, with each side sticking to competing demands for a deal that could shape the Middle East for generations.
Trump's latest post followed the largest co-ordinated strike of the war by Israel on Wednesday that killed more than 250 people in Lebanon, prompting a warning from Iran's lead negotiator that Israel's ramping up its parallel war and Washington's insistence on Tehran abandoning its nuclear ambitions could jeopardise talks to forge a permanent peace deal.
"In such a situation, a bilateral ceasefire or negotiations were unreasonable," Iran's parliament speaker Mohammed Bager Qalibaf, said in a statement.
Oil prices climbed on Thursday, with investors concerned about the fragility of the truce and elevated geopolitical risks over Middle East supply, with doubts that restrictions in the Strait of Hormuz would soon ease.
Israel's blitz on Wednesday raised questions about regional truce efforts, with conflicting messages on the scope of the ceasefire and sharply contrasting agendas for peace talks set to start on Saturday in Pakistan.
The United States and Israel say Lebanon is not included in the agreement, while Iranian President Masoud Pezeshkian has said a halt in hostilities in Lebanon was an essential condition of Tehran's deal with Washington.
The Iran-backed Hezbollah said it fired rockets early on Thursday at a small kibbutz, citing what it described as Israel's ceasefire violations, warning of more until there was a halt in "Israeli-American aggression".
Pakistan's foreign ministry condemned the Israeli operations on Thursday, which it said "undermine international efforts to establish peace and stability".
Iran's delegation for the talks was due to arrive in Islamabad on Thursday night.
"Despite scepticism of Iranian public opinion due to repeated ceasefire violations by Israeli regime ... Iranian delegation arrives tonight in Islamabad for serious talks based on 10 points proposed by Iran," Ambassador Reza Amiri Moghadam said in a post on X.
There was scant sign that the Strait of Hormuz was open in any meaningful way since the truce agreement, with Iran still asserting its control over the vital artery, a conduit for a fifth of the world's oil and liquefied natural gas supply, and demanding tolls for safe passage.
Tehran's newly demonstrated ability to cut off Gulf energy supplies through its grip on the strait, despite decades of massive US military investment in the region, shows how the conflict has already altered power dynamics in the Gulf.
Iran's Revolutionary Guards navy posted a map showing alternative routes in the Strait of Hormuz to help ships avoid naval mines, the Iranian news agency ISNA said.
British Foreign Secretary Yvette Cooper will say on Thursday that shipping through the strait must be toll-free, countering a push by Iran to control a channel long treated as an international waterway.
"The fundamental freedoms of the seas must not be unilaterally withdrawn or sold off to individual bidders. Nor can there be any place for tolls on an international waterway," Cooper will say in a foreign policy speech in London, according to advance extracts.
