The status of a two-week-old ceasefire, due to have expired on April 22, remained unclear.
In a sharp about-face hours after threatening renewed violence, Trump made what appeared to be a unilateral announcement on Tuesday that the US would extend a ceasefire until it had discussed an Iranian proposal in peace talks to end the two-month-old war.
But Iranian officials did not say they had agreed to any extension and criticised Trump's decision to maintain the US Navy blockade of Iran's trade by sea, itself considered by Iran an act of war.
Iran's parliament speaker and lead negotiator Mohammad Baqer Qalibaf said a full ceasefire made sense only if the blockade was lifted.
Reopening the Strait of Hormuz, the slender choke point that carried a fifth of the world's oil trade before the war, was impossible with such a "flagrant breach of the ceasefire", Qalibaf said on social media.
"You did not achieve your goals through military aggression and you will not achieve them by bullying either," he wrote.
Trump again backed away at the last moment from his repeated threats to bomb Iran's power plants and other civilian infrastructure, which the United Nations and others warn would violate international law.
But little progress has been made in ending the war that started with joint US-Israeli attacks on Iran on February 28.
That leaves the two sides in a holding pattern with the crucial Strait of Hormuz still effectively shut, straining economies across the world.
Thousands of people have been killed across the Middle East, mostly in Iran and Lebanon, where Iran-allied Hezbollah joined the fighting against Israel.
The Islamic Revolutionary Guard Corps seized two vessels and escorted them to Iranian shores, according to the shipping companies and Iran's Tasnim news agency.
The Revolutionary Guards accused the ships it had seized, the Liberia-flagged Epaminondas and Panama-flagged MSC Francesca, of operating without required permits and tampering with their navigation systems.
A third, Liberia-flagged container ship was fired upon in the same area but was not damaged and resumed sailing.
White House press secretary Karoline Leavitt said told Fox News that since the ships were not US or Israeli vessels the seizure was not a violation of the ceasefire.
The US military said on Wednesday it had directed more than 30 ships to turn around or return to port as part of the US blockade against Iran.
Brent, the international crude oil benchmark, remained above $US100 a barrel on Thursday, having hit triple figures a day earlier for the first time in two weeks.
In his Tuesday announcement, Trump said the US had agreed to a request by Pakistani mediators "to hold our attack on the country of Iran until such time as their leaders and representatives can come up with a unified proposal ... and discussions are concluded, one way or the other".
Pakistan, which has acted as a mediator, was still trying to bring the sides together after both failed to show up for tentatively scheduled talks in Islamabad on Tuesday before the two-week-old ceasefire was due to expire.
A first session of peace talks between Iran and the US in Islamabad 11 days ago produced no agreement.
Trump wants Iran to give up highly enriched uranium and forgo further enrichment to prevent it building a nuclear weapon.
Iran says it has only a peaceful civilian nuclear program, and wants sanctions lifted, reparations for damage and recognition of its control over the strait.
