US President Donald Trump said he hoped to reach a "great deal" to end the war but he did not want to extend the ceasefire, and said the US military was "raring to go" if negotiations were not successful.
Vice President JD Vance, due to lead the US delegation, had yet to board a flight for Islamabad, where the centre of the city has been locked down and a luxury hotel has been cleared out to host the talks.
A White House official said Vance would participate in additional meetings in Washington DC on Tuesday.
By the evening on Tuesday, the spokesman for Iran's foreign ministry, Esmaeil Baghaei, told state television Iran had still yet to decide whether to attend.
He described the boarding of the tanker, as well as the seizure of a separate cargo ship on Sunday, as "piracy at sea and state terrorism," which he said called into question the seriousness of the US in negotiating.
"The aggression against Iranian ships and the continued pressure indicate the continuation of the opposing side's contradictory behaviour," Baghaei said.
Pakistan's Information Minister Attaullah Tarar said in a post on X that Pakistan was still waiting for Iran's reply to its invitation: "Pakistan as the mediator is in constant touch with Iranians and pursuing the path of diplomacy and dialogue."
Asked about the possibility of extending the truce, Trump told CNBC: "I don't want to do that. We don't have that much time."
"I expect to be bombing because I think that's a better attitude to go in with," he added.
"But we're ready to go. I mean, the military is raring to go."
Iranian government spokeswoman Fatemeh Mohajerani said: "We do not want to be attacked again but if such attacks occur, we will definitely respond more firmly than before," according to the state news agency IRNA.
The US military said its forces had boarded a tanker under sanctions for trading with Iran, the Tifani, in an operation that took place "without incident".
The vessel last reported its position on Tuesday morning as near Sri Lanka in the Indian Ocean, according to MarineTraffic tracking data.
It was close to fully loaded with two million barrels of crude and had signalled Singapore as its destination.
Iranian officials have publicly cited a US blockade of their ships as a reason not to negotiate.
On social media, Trump said Iran had carried out numerous violations of the ceasefire, without giving further details.
He told CNBC that the blockade had been a success and the US was in a strong position to end up with a "great deal".
The precise timing of the ceasefire's end has been a source of confusion.
Trump initially announced it would last two weeks from the evening of Tuesday, April 7 in Washington DC.
But he said this week that it runs until the evening of Wednesday, April 22, effectively giving it an extra 24 hours.
A Pakistani source involved in the talks also said it would expire at 8pm in Washington DC on Wednesday, which is 3.30am on Thursday in Iran (10am on Thursday AEST).
Pakistan has been preparing to host the talks despite the uncertainty.
Nearly 20,000 security personnel have been deployed across Islamabad, officials said.
