The next 24 hours may prove to be the most critical of President Donald Trump's now six-weekwar in Iranas the two sides are meeting for the first time since the fighting started.
Vice President JD Vance led a delegation of U.S. officials for meetings with Iranians being mediated by Pakistan in Islamabad on Saturday that continued into the early morning hours, according to the White House.
Asked about the state of ongoing negotiations between the U.S, Iran and Pakistan, a senior White House official told ABC News: "15 hours and counting!"
The statement came shortly after 7:30 p.m. ET on Saturday, which is 4:30 a.m. in Islamabad.
Both sides met first with Pakistani leaders before the historic sitdown Saturday, the highest level face-to-face meeting between the U.S. and Iran in 47 years.
In addition to Vance, special envoy Steve Witkoff and the president's son-in-law, Jared Kushner took part in the talks with top Iranian officials, including the speaker of the Iranian Parliament.
Earlier Saturday, a senior White House official confirmed that the U.S., Pakistan and Iran were holding a trilateral face-to-face meeting, per the press pool traveling with the vice president.
In a phone interview with NewsNation Saturday morning, President Donald Trump says peace talks between U.S. and Iran have begun, and that he has "no idea" how they will go.
Trump sidestepped when asked if he believes the Iranians are negotiating in good faith, responding: "I’ll let you know that in a very short period of time, won’t take long."
As he left Friday, Vance said he expected "positive" negotiations. But he had a warning for the Iranians, too.
"If they're going to try to play us, then they're going to find the negotiating team is not that receptive," he told reporters.
President Trump on Friday afternoon, when asked about the upcoming talks, said, "I think it's going to go pretty quickly. And if it doesn't, we'll be able to finish it off one way or the other."
As talks begin, which side has more leverage in the standoff is an open question.
The U.S negotiating team also includes a host of officials from the State Department, Department of Defense and National Security Council, the White House said.
"I wish him luck. He's got a big thing. We'll find out what's going on," Trump said of Vance.
The ceasefire announced by Trump on Tuesday night has been fragile, and both sides have accused the other of not meeting preconditions for the talks.
On Day 1 of the truce, and in the days since, Israel has stepped up attacks against Hezbollah, an Iranian proxy, in Lebanon, outraging Iran and leading to accusations the terms had been breached.
Vance told reporters Thursday it was a "legitimate misunderstanding" and that the U.S. never included stopping Israeli strikes on Lebanon in the ceasefire deal. Iran's parliament speaker, Mohammad Bagher Ghalibaf, reiterated on Friday that a ceasefire in Lebanon must occur before talks begin.
JD Vance to head US negotiations with Iran: What comes next?
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Israel's ongoing offensive in Lebanon "is going to create major stress on" talks, said Syed Mohammad Ali, a lecturer at Johns Hopkins University and analyst of Pakistani politics.
Whether the U.S. would put pressure Israel to temper its campaign is unclear before the U.S. and Iran come to the table, Ali said.
President Trump said Thursday that Israeli Prime Minister Benjamin Netanyahu told him Israel would begin to "low-key" its offensive.
Ghalibaf, the Iranian parliament speaker, also said in a post on X on that "the release of Iran's blocked assets" (funds frozen by sanctions in banks around the world) was also agreed upon before the talks, and the U.S. has yet to do that.
U.S. officials, mediators and even the Iranians have not previously indicated this was a precondition. The White House has not responded to a request for comment. President Trump has long been critical of former President Barack Obama for returning more than a billion dollars in frozen Iranian money as part of his 2015 Iran nuclear deal.
Meanwhile, Trump has scolded Iran for not allowing the free flow of ships through the Strait of Hormuz -- a condition he set for the ceasefire and something he said must be met before any talks.
"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 president posted on his social media platform on Friday afternoon. "The only reason they are alive today is to negotiate!"
But the Iranians have proven so far they can weaponize the flow of commerce through the strait, and it has put real pressure on the Americans to negotiate.
Before Trump started the war, oil flowed freely there. Now, Americans are paying, on average, more than $4 a gallon on gas, andinflation surgedto the highest rate in nearly two years in the month since the war began.
Even if the two sides can get past these hurdles and begin talks, the issues on the table are enormous.
The U.S. wants guarantees Iran will permanently end enriching uranium that could be used for a nuclear weapon, and that Iran hands over or destroy its existing stockpile -- two demands Trump cited as reasons for going to war.
Trump also wants limits on Iran's missile production and for Tehran to end its support for what the U.S. calls terrorist proxy groups in the region.
The U.S. has not formally released its set of demands, but President Trump said on Wednesday that his team had presented Iran with a 15-point plan, saying many demands "have already been agreed to."
Meanwhile, the Iranians are demanding an end to economic sanctions, control over the flow of commerce through the strait in perpetuity, a permanent end to the U.S. and Israeli bombings and for American forces to leave the region.
Vance, who has long opposed U.S. military intervention in the Middle East and other countries, made his reservations about entering a conflict with Tehran known internally before Trump ordered the first set of U.S. strikes on Feb. 28,ABC News has previously reported.
Trump, who has acknowledged he and Vance were "philosophically different" on the matter, said in an interview on Friday with the New York Post that Vance, who has limited diplomatic experience, is doing a "very good job" and that he has "nothing to prove."
And in yet another threat to Tehran, Trump told the Post that U.S. military vessels are being prepared and reloaded with ammunition in case the peace talks in Islamabad fail.
"And if we don't have a deal, we will be using them, and we will be using them very effectively," Trump reportedly said.
ABC News' Emily Chang contributed to this report.