An Iranian delegation led by parliamentary speaker Mohammad Bagher Ghalibaf is also in the city, having arrived overnight at an air base near the capital and disembarked to embrace Pakistan's powerful army chief Asim Munir.

Munir, who shares a personal rapport with US President Donald Trump, also greeted Vance, escorting him down a red carpet at the Nur Khan air base, where US special envoy Steve Witkoff and Trump's son-in-law Jared Kushner were already waiting.

It was unclear what time the talks would get underway, though Iranian news agencies reported Tehran's delegation was on its way to a meeting with Pakistani Prime Minister Shehbaz Sharif at 1:00 pm local time.

The warring parties still appeared to be far apart on key issues, including the opening of the strategic Strait of Hormuz, and made no attempt to hide their mutual suspicion.

"Our experience in negotiating with the Americans has always been met with failure and broken promises," Ghalibaf said shortly after landing, according to Iran's state broadcaster.

Foreign Minister Abbas Araghchi, who is also part of the delegation, told his German counterpart in a call on Saturday that "Iran enters negotiations with complete distrust due to repeated breaches of commitments and betrayals by the United States", the Tasnim news agency reported.

Vance said before leaving the US that if the other side was "willing to negotiate in good faith, we're certainly willing to extend the open hand".

- 'Make or break' -

But "if they're going to try to play us, then they're going to find the negotiating team is not that receptive", he added.

After arriving in Islamabad, Vance made a brief stop at the US embassy, before heading to his hotel, according to his office.

The ceasefire is already under strain, notably from Israel's continued strikes in Lebanon, which Iran and Pakistan insist is covered under the current truce.

Prime Minister Sharif, whose country's down-to-the-wire mediation got both sides to the negotiating table this week, said talks would not be easy.

"An even more difficult stage lies ahead," he said, referring to efforts to permanently end fighting that began with US-Israeli strikes on Iran on February 28, sparking Iranian retaliation against Israel and across the Gulf.

"This is that stage which, in English, is called the equivalent of 'make or break.'"

Pakistani Foreign Minister Ishaq Dar "expressed the hope that parties would engage constructively", and reiterated Islamabad's willingness to facilitate a "lasting and durable solution to the conflict".

Iranian state television made only a brief mention of the talks in its first morning news programme, with its second report dedicated to volunteers signing up to defend Iran in the event the war resumed.

- Islamabad plays host -

Iran -- which brought a more than 70-member delegation to Pakistan -- has insisted on the truce covering Lebanon and on the unfreezing of its assets for the Islamabad talks to go ahead, neither of which has materialised so far.

On the US side, President Donald Trump demanded the opening of the Strait of Hormuz as a condition for the two-week ceasefire.

The strait, through which one-fifth of the world's crude passes, has not reopened to normal traffic, however, and Trump vowed on Friday to have it open soon "with or without" Iran's cooperation.

He added his top priority at the Islamabad talks was to ensure the Islamic republic had "no nuclear weapon. That's 99 percent of it."

Security was tight in the Pakistani capital on Saturday, with a heavy police and paramilitary presence on the streets and road diversions around the "red zone" where government and diplomatic buildings are located.

It was not known whether the two sides would meet face-to-face, or whether they would mirror an indirect format used in Oman-mediated talks before the war.

Pakistan has formulated a team of subject matter specialists to facilitate the two sides in negotiations on navigation, nuclear and other key matters, a diplomatic source familiar with the matter told AFP.

The negotiations will be closely watched by other key regional players, with Egypt and Turkey having helped with mediation, along with China, all of which Pakistan was still coordinating closely with for the talks, the source said.

Beijing has been sought as a possible guarantor of any lasting agreement, official sources have said, with Trump confirming to AFP that China helped get Tehran to the negotiating table.

It was not clear whether China would have any direct presence during the talks or would be willing to take on a formal role.

- Violence in Lebanon -

Complicating the path to a permanent ceasefire was Israel's assertion that the current truce does not cover Lebanon.

Israeli air strikes continued in Lebanon on Friday against Iran-backed Hezbollah despite the Iranian demand that they be halted.

Israel's ambassador to the United States, Yechiel Leiter, said his country would hold discussions with Lebanon's government in Washington next week but would not discuss a ceasefire with Hezbollah.

The militant group said overnight that it had carried out drone and rocket attacks on northern Israel, as well as on Israeli forces in southern Lebanon.

In Tehran, a 30-year-old resident told AFP he was skeptical negotiations would be successful, describing most of what Trump says as "pure noise and nonsense."

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