However, a senior Iranian official told Reuters on Monday that no decision had been made and Iranian Foreign Minister Abbas Araqchi said "continued violations of the ceasefire" by the US are a major obstacle to continuing the diplomatic process.
Araqchi told his Pakistani counterpart Ishaq Dar that Iran, while taking all aspects of the matter into account, had yet to decide how to proceed further.
On Monday night, Iran's top negotiator Mohammad Baqer Qalibaf accused US President Donald Trump on X of increasing pressure on Tehran through the blockade and ceasefire violations, saying Iran rejects negotiations under threat.
The two-week ceasefire in a conflict that has killed thousands of people and roiled the global economy, particularly energy markets, is set to expire this week.
It had appeared to be in jeopardy after the US said it had seized an Iranian cargo ship that tried to run its blockade and Tehran vowed to retaliate.
In the Islamabad talks, Trump is eager for a deal that would help avoid another surge in oil prices and plunge in stock markets.
Iran hopes to leverage its control of the Strait of Hormuz, a crucial shipping channel for global energy supplies, to get an agreement that prevents a resumption of the war, and allows financial relief from long-running sanctions and some breathing room for its nuclear program.
Adding to the uncertainty, Vice President JD Vance remained in the US on Monday, a separate source told Reuters, denying reports he was already on his way to Pakistan for talks.
Trump announced the two-week ceasefire with Iran on April 7, and has not specified precisely when it ends.
He said on social media that he believed his administration's nuclear deal with Iran would be better than a 2015 international agreement reached after years of negotiations under then-president Barack Obama, a Democrat.
Trump withdrew from that agreement - which had been vehemently opposed by congressional Republicans and Israeli Prime Minister Benjamin Netanyahu - in 2018, during his first term as president.
It was unclear what kind of agreement could be reached in just a few days of talks, but the Republican US president predicted a quick result.
The US was hoping to start negotiations in Pakistan shortly before the ceasefire expires.
However, Iranian President Masoud Pezeshkian said that "unconstructive & contradictory signals from American officials carry a bitter message; they seek Iran's surrender."
"Iranians do not submit to force," he added on X.
The US has maintained its blockade of Iranian ports, while Iran lifted and then reimposed its own blockade of the Strait of Hormuz, which typically handles around one-fifth of the world's oil and liquefied natural gas supply.
Trump on Sunday warned the US would destroy every bridge and power plant in Iran if it rejected his terms, continuing a recent pattern of such threats.
Iran has said that if the US were to attack its civilian infrastructure, it would strike power stations and desalination plants in its Gulf Arab neighbours.
Pakistan geared up to host the talks despite uncertainty over whether they would go ahead. Nearly 20,000 security personnel have been deployed across Islamabad, a government official and a security official said.
