JD Vance has warned Iran not to “try and play” the US at talks planned for Saturday in Islamabad, while Tehran said it would not take part until Israel stopped its bombing of Lebanon.

The US vice-president made the comments as he boarded a plane to Pakistan for negotiations that could determine whether a ceasefire holds or the war on Iran resumes with grave implications for the global economy.

With hours to go before the talks were scheduled to start, doubts remained as to whether they actually would.

Meanwhile, it was announced Friday that Lebanon and Israel had agreed to meet in Washington DC on Tuesday to discuss a ceasefire and to set a date to begin talks. The date of the meeting was set during a phone call between Lebanon’s ambassador to the US, Nada Hamadeh Mouawad; Israel’s ambassador to the US, Yehiel Leiter; and the US ambassador to Lebanon, Michael Issa. The conversation on Tuesday will be mediated by the US and take place at the state department.

Mohammad Bagher Ghalibaf, Iran’s parliamentary speaker and the co-leader of the country’s delegation, said on X on Friday: “Two of the measures mutually agreed upon between the parties have yet to be implemented: a ceasefire in Lebanon and the release of Iran’s blocked assets prior to the commencement of negotiations. These two matters must be fulfilled before negotiations begin.”

It was unclear on Friday evening whether Ghalibaf and Abbas Araghchi, Iran’s foreign minister, were still planning to fly to Islamabad to lead their delegation. It was reported last month that Israel had taken Ghalibaf and Araghchi off the target list of its bombing campaign at Washington’s request.

Donald Trump fuelled the uncertainty by saying US forces were rearming and ready to return to the attack if the negotiations failed.

“We have a reset going. We’re loading up the ships with the best ammunition, the best weapons ever made – even better than what we did previously and we blew them apart,” the US president told the New York Post. “And if we don’t have a deal, we will be using them, and we will be using them very effectively.”

Later on Friday, Trump followed up his threats with a post on his own social media site declaring: “The Iranians don’t seem to realize they have no cards, other than a short term extortion of the World by using International Waterways. The only reason they are alive today is to negotiate!”

The Iranians and the Pakistani mediators say the two-week ceasefire agreement struck by the US and Iran on the night of 7 April included Lebanon. Trump and Israel’s prime minister, Benjamin Netanyahu, say this is not the case, and Israel has continued to bomb Lebanon in its offensive against Hezbollah, Iran’s closest ally in the region, even after Netanyahu said he was ready to start peace talks with the Lebanese government.

More than 300 Lebanese citizens have been killed by Israeli bombing since the ceasefire started. The country’s president, Joseph Aoun, said on Friday that 13 state security personnel had been killed in an Israeli strike on a government building in the southern city of Nabatieh.

Trump has accused Iran of doing a “very poor job” of allowing oil to go through the strait, adding in an overnight social media post: “That is not the agreement we have!”

The oil price spike caused by the US-Israeli attack on Iran on 28 February, and Iran’s response by closing the strait of Hormuz to oil tankers and other shipping, is a direct political threat to the president before congressional elections in November.

Despite the uncertain outlook for the ceasefire, Vance’s tone was generally optimistic as he boarded Air Force Two. “We’re looking forward to the negotiation. I think it’s going to be positive,” Vance said. “If the Iranians are willing to negotiate in good faith, we’re certainly willing to extend the open hand.”

But, he added: “If they’re going to try and play us, then they’re going to find that the negotiating team is not that receptive.”

Trump’s all-purpose international negotiator, Steve Witkoff, was also expected to be in the US delegation, as was the president’s son-in-law, Jared Kushner. Witkoff and Kushner took part in talks with Iranian negotiators prior to the US-Israeli attack, which had been focused on Iran’s nuclear and missile programmes.

The Omani mediators of those talks, as well as UK government observers, believed meaningful progress had been made and were expecting another round of negotiations when the US and Israel launched their surprise attack on 28 February.

After more than five weeks of bombing, the campaign has killed Iran’s supreme leader, Ali Khamenei, but failed to dislodge the Islamic regime in Tehran. It has also inflicted heavy losses on Iran’s armed forces but they were still able to threaten shipping in the Hormuz strait and cut off the flow of a fifth of the world’s oil and liquified fossil gas.

The Islamabad talks are expected to focus on the reopening of the strait, as well as the future of Iran’s nuclear programme and the prospect of sanctions relief. Iran says it will also demand reparations for war damage. According to the Washington Post, Vance’s delegation intends to demand the release of Americans detained in Iran.

Advance teams from the US and Iran reportedly began to take up rooms on Friday in the five-star Serena hotel in central Islamabad, with Pakistani officials relaying messages between the two camps. Officials from Saudi Arabia and other Gulf countries were also arriving to represent their interests.

A 2-mile (3km) security perimeter was set up around the hotel by Pakistani security services, the city centre was locked down and a public holiday declared for Pakistan’s highest-level mediation effort on the world stage in recent times.

Hezbollah did not comment on news of the direct negotiations between the Lebanese and Israeli governments, despite the group’s historical opposition to any contact with Israel. The head of the armed group, Naim Qassem, in a written statement published on Friday afternoon, called on the Lebanese government to “cease making gratuitous concessions”, without making explicit mention of the negotiations.

“We will not accept a return to the status quo, and we call upon those in authority to cease making gratuitous concessions,” Qassem said, vowing to keep fighting and to “expel the occupier”.

The Lebanese army deployed additional soldiers across Beirut, with a strong presence by the prime minister’s office, as the government sought to implement its decision to allow no arms outside state control in the country’s capital city. The decision was taken after Israel’s attack on Wednesday, which left at least 303 people dead.

Fighting has continued in south Lebanon despite the upcoming negotiations and Hezbollah claimed to have struck Israeli soldiers on the outskirts of Bint Jbeil.

Bint Jbeil carries historical and strategic significance because Hezbollah managed to keep Israeli forces out of the town in the 2006 war – earning it the moniker of the “capital of resistance”. The area is also key to controlling the central area of southern Lebanon because it sits at the crossroads of neighbouring towns and hills.

Hezbollah launched volleys of rockets at Israel throughout Friday.

Israel carried out airstrikes across Lebanon, killing 13 government security officers in an attack near the provincial government’s headquarters in Nabatieh, according to Lebanon’s state security agency. It was the highest number of Lebanese security forces killed by Israel so far.

Lebanon’s government is not a party to the Hezbollah-Israel war but Israeli strikes have killed Lebanese soldiers over the course of the conflict.