A first wave of United Nations military observers landed in Muscat on Monday and Pakistani, Egyptian and Saudi liaison officers began round-the-clock shifts at the Strait of Hormuz Verification Cell, racing to stand up the machinery of the Islamabad ceasefire less than 48 hours before it takes formal effect at midnight Wednesday Greenwich Mean Time.

The deployment, foreshadowed in the joint statement issued in the Pakistani capital on Sunday, marked the operational phase of a diplomatic effort that has consumed the region for two weeks and put a halt within reach of a war that has killed roughly 370 American service members and displaced more than three million people across Iraq, Lebanon and Iran since fighting began in early March.

UN Secretary-General Antonio Guterres, in a statement issued from New York early Monday, said an initial complement of 84 unarmed observers drawn from twelve troop-contributing countries would arrive in Oman by Tuesday evening, with a forward team already on the ground at a Royal Air Force of Oman facility outside Seeb International Airport. A separate maritime cell, hosted aboard a Norwegian-flagged auxiliary vessel and supplemented by a Pakistani frigate, was to begin patrols of the Strait of Hormuz at the moment the ceasefire took effect.

“The pieces are being put in place that did not exist a week ago,” Guterres said. “The hours between now and Wednesday will be the test of whether the parties intend to use them.”

The Strait of Hormuz Verification Cell, formally chaired by Omani officers and staffed by liaison officers from Pakistan, Saudi Arabia, Egypt, Qatar and the United Nations, has begun receiving classified daily summaries from the U.S. Fifth Fleet in Bahrain and from Iranian naval headquarters in Bandar Abbas, two officials briefed on the arrangement said. The cell’s commander, Brigadier Yousuf al-Harthi of the Royal Army of Oman, took up his post Sunday night and held his first videoconference with Iranian, U.S. and Israeli liaison desks before dawn.

“We are operating to a schedule that was designed to be uncomfortable,” al-Harthi said in a brief statement read by a spokesman at the Diwan in Muscat. “Each party knows what is expected of it. Each party knows what the others have agreed to. The architecture is in place.”

At Islamabad, where Pakistani Prime Minister Shehbaz Sharif and Foreign Minister Ishaq Dar have kept the mediation venue formally open through the standdown window, officials said the principal diplomatic work of the next two days would be sequencing. Three diplomats familiar with the choreography described a meticulously rehearsed handoff in which Israeli aircraft would terminate offensive sorties no later than 23:00 GMT Tuesday, Iranian and Iraqi-launched munitions would cease at the same hour, and Houthi forces in Yemen would observe a “silent forty-eight hours” beginning Monday at 16:00 GMT, monitored from the maritime cell. A 72-hour confidence-building window would then begin at 00:00 GMT Wednesday, during which any single launch or strike would trigger the framework’s “cooling clause,” suspending the ceasefire for 24 hours rather than collapsing it.

A senior State Department official, speaking on condition of anonymity to describe the implementation arrangements, said Washington had quietly extended its monitoring agreement with the Muscat cell to include real-time satellite feeds from the Defense Department’s Gulf surveillance constellation, on the condition that the feeds remain in U.S. custody and be shared only through Omani channels. “We are not putting American intelligence into a multilateral room,” the official said. “We are making sure the multilateral room has eyes.”

Israeli officials, who have remained outside the Islamabad venue and communicated through an American-led liaison cell in Riyadh, were said by two people familiar with the talks to have given Washington a private commitment that Israeli aircraft would not strike Iranian territory after the 23:00 Tuesday cutoff except in the event of a verified launch toward Israeli territory. The Prime Minister’s office issued no public statement, but a senior aide to Defense Minister Yoav Gallant told Israeli Army Radio Monday morning that the Israel Defense Forces were “in a posture of disciplined readiness” and that no decision had been taken to extend the strike campaign into the standdown window.

In Tehran, Foreign Minister Abbas Araghchi convened a meeting of senior commanders and the head of the IRGC Aerospace Force at the Pasteur Street compound on Sunday night, according to Iranian state media, which described the gathering in unusually procedural terms. State television aired footage of Araghchi being briefed on the cell’s communications protocols and quoted him telling commanders that “the next forty-eight hours are not an interval; they are part of the agreement.”

Diplomats in Islamabad and Muscat acknowledged that the highest residual risk lay in unilateral action by Houthi forces in Yemen, who have not been formally party to the talks and whose compliance Iran has agreed to secure through a parallel sub-track in Doha. Qatari Foreign Minister Mohammed bin Abdulrahman al-Thani, who has chaired the Houthi sub-track for the past nine days, said in a televised interview that Sanaa had given “verbal undertakings consistent with the framework” but that any final assurance would only be measurable in the absence of launches.

“The Red Sea is the variable that has not yet stopped moving,” said Layla Hassan, a Beirut-based regional analyst at the Levant Policy Forum. “Everyone in this process has now agreed to be tested by silence in places where they have only been heard through noise.”

A separate annex on the prisoner exchange, set to take place in Doha on April 18, was finalized over the weekend by negotiators from Qatar, Switzerland and the International Committee of the Red Cross, two officials familiar with the document said. Under the terms, Iran is to release approximately 40 detained foreign nationals — among them four U.S. citizens, two dual-national journalists and a number of European businesspeople — together with the remains of 11 American service members. The United States and Israel are to release a smaller number of Iranian nationals and Hezbollah-affiliated detainees, with no tribunal envisioned. The ICRC will conduct medical screening of all released detainees at a transit facility outside Doha.

European foreign ministers, meeting by secure call Monday morning, endorsed a French proposal to provide initial funding for the UN observer mission and to underwrite a Brussels donor conference for reconstruction once the ceasefire’s 30-day verification window closes. EU foreign policy chief Kaja Kallas said the bloc would commit 240 million euros to the observer mission’s first six months. Russia, which has not played a visible role in the talks, said it would not block any UN Security Council resolution required to authorize the deployment.

Strikes continued through Sunday night and into Monday. Iranian state media reported that Israeli aircraft had hit a centrifuge component facility near Natanz overnight in what officials in Jerusalem described as the final strike of the campaign. Iran fired a salvo of medium-range missiles toward Israeli air bases in the Negev shortly before dawn; most were intercepted, according to U.S. Central Command, though one fragment damaged a fuel depot at Nevatim, causing no casualties. Houthi forces did not launch against Red Sea shipping for a third consecutive day, the longest such pause since mid-March.

Pakistani officials said the talks compound in Islamabad would remain staffed through the ceasefire hour and that mediators expected to issue a final joint statement on Thursday confirming the standdown window had held. Dar said additional steps, including the timing of a UN Security Council briefing and the formal opening of the Doha prisoner exchange channel, would be announced as the parties signaled readiness.