DOHARAK, Turkey — A column of 142 trucks loaded with diesel generators, dialysis filters, infant formula and pallets of insulin stretched for nearly two kilometers along the road leading to Iran’s Bazargan crossing on Tuesday afternoon, engines idling in the thin alpine wind as customs officials, World Health Organization staff and Turkish Red Crescent dispatchers walked the line one last time. None of the cargo was permitted to move. Not yet.

With less than 36 hours before the Islamabad ceasefire is scheduled to take effect at 00:00 GMT Thursday — midnight Wednesday GMT — international humanitarian agencies were racing on Tuesday to pre-position relief convoys at half a dozen border points around the war zone, betting that the truce would hold long enough to open at least a narrow window for the largest single-day aid surge into Iran, Iraq, Lebanon and Yemen since the conflict began in early March.

The U.N. Office for the Coordination of Humanitarian Affairs, which is coordinating the staging effort from a temporary cell in Amman, said in a Tuesday morning update that 41 convoys totaling roughly 1,180 trucks had been assembled at six crossings: Bazargan and Sero on the Turkish-Iranian border, Haji Omeran on the Iraqi Kurdistan-Iran border, Trebil on the Jordan-Iraq route, the Masnaa crossing between Syria and Lebanon, and the Wadia port-of-entry into Yemen from Saudi Arabia. A further 400 trucks were on standby in Dubai, Doha and Larnaca, the office said, awaiting flight slots or sea lanes that remained constrained by lingering insurance restrictions.

“This is the largest pre-positioning we have ever attempted in this part of the world, and we have a window measured in days, not weeks, to make it count,” said Reena Ghelani, the U.N. emergency relief coordinator’s deputy, speaking by video link from Amman. “If the ceasefire holds on Thursday morning, we will move. If it slips, we will sit in this dust and wait. That is the honest picture.”

The staging operation reflected the broader recalibration underway across the humanitarian system as the Islamabad accord’s deadline approached. Through the war’s six weeks, sustained aid access had been impossible across most of the conflict zone. Iran sealed its airspace in mid-March, halting all but a handful of medical-evacuation flights coordinated with the Swiss government and the International Committee of the Red Cross. Iraq’s main supply corridor from Jordan was repeatedly closed during U.S. strikes on Iranian-aligned militias near Trebil. Yemen’s Hodeidah port operated at one-third capacity for much of the war as Houthi forces and Saudi-led coalition aircraft contested the Red Sea approaches.

By Tuesday, the pattern was beginning to shift. Iran’s civil aviation authority announced overnight that Mehrabad and Imam Khomeini airports in Tehran would reopen to designated humanitarian flights starting at 06:00 local time Thursday, with corridors deconflicted through the Pakistani-hosted liaison cell in Islamabad. The Iraqi government said the Trebil crossing would operate around the clock for at least two weeks beginning Thursday. Lebanon’s caretaker prime minister, Nawaf Salam, said in a Beirut briefing that the Masnaa crossing was already running at expanded hours for medical cases.

In Erbil, ICRC regional director Olivier Marchand said his organization had 28 trucks staged at Haji Omeran, with a priority manifest weighted toward dialysis consumables, oxygen concentrators, blood products and prefabricated burn-unit modules destined for hospitals in Urmia, Tabriz and Kermanshah. The agency had also chartered two cargo aircraft now positioned at Erbil International Airport, he said, with takeoff slots reserved for Thursday morning if Iran’s air corridor opens as scheduled.

“We have hospitals in northwestern Iran that have been rationing dialysis at 40 percent of normal cycles for four weeks,” Marchand said. “Patients are dying who would not have died in a hospital with full supplies. The math of this is not abstract.”

The pre-positioning has been complicated by a thicket of operational and political details that aid officials said would matter enormously on Thursday morning. Insurance underwriters were still finalizing war-risk clauses for the post-ceasefire window, with several agencies relying on sovereign indemnification from Norway, Switzerland and Qatar to keep convoys moving in the first 72 hours. The U.S. Treasury’s Office of Foreign Assets Control issued a clarifying notice Tuesday confirming that humanitarian transactions with Iranian counterparties, including the Iranian Red Crescent and a list of named hospitals, would not be considered sanctionable through May 31, a window aid groups had pressed hard to secure.

A senior State Department official, speaking on condition of anonymity to describe internal planning, said Washington was “leaning very hard forward” on humanitarian access in the first days after the ceasefire and had urged Israel and Iran through the Islamabad mediators to refrain from restricting overflight routes that would shorten supply lines. The official said the administration had also authorized the U.S. Air Force’s 386th Air Expeditionary Wing to provide tactical airlift support to ICRC and WHO convoys “if requested and if conditions permit.”

Civilian anticipation, aid workers said, was visible at every staging point. In Doharak, the Turkish village a few kilometers from Bazargan, residents had set up trestle tables along the convoy route handing out flatbread, hot tea and small bags of dried fruit to drivers who had been on the road since Monday. A Turkish Red Crescent dispatcher, Selin Aydin, said her team had logged 78 separate inquiries since Sunday from Iranian families in Tabriz asking whether the convoys were carrying letters or parcels from relatives in Turkey and Europe. They were not, she said, but the agency had decided to add a humanitarian-mail pouch to each truck for the return trip.

“People want to know that something is moving in their direction,” Aydin said. “Even one truck arriving means the door has opened. After six weeks, that matters as much as what is inside the truck.”

The staging effort has also exposed the limits of what a ceasefire can do quickly. In southern Lebanon, where the Lebanese Red Cross and Médecins Sans Frontières had assembled mobile clinics near Tyre and Nabatieh, MSF country director Camille Pereira said unexploded ordnance and damaged road infrastructure would constrain the agency’s reach into border villages “for weeks, possibly months.” In Yemen, U.N. officials said the Hodeidah port would require a separate deconfliction arrangement, still under negotiation in Muscat, before bulk-grain shipments could resume at scale.

UNHCR’s regional bureau in Amman said it expected the first significant returns of displaced Iraqis from Jordan and Iranian Kurds from Iraqi Kurdistan within ten to fourteen days of a sustained ceasefire, though officials cautioned that families with damaged or destroyed homes would likely remain in host communities through the summer. The agency reiterated that returns would be voluntary and that host governments had committed not to compel movement.

For now, on the Turkish-Iranian frontier, the convoy waited. A WHO logistics officer working the line, Tarek Mansour, said the agency had drilled the crossing sequence three times since Saturday: customs paperwork, deconfliction call to Islamabad, lead-vehicle dispatch at five-minute intervals, medical escorts staged behind every twelfth truck. “We can be across the border and on the road to Tabriz within four hours of the ceasefire taking effect,” Mansour said, “provided no one fires a last shot we have not planned for.” OCHA said a consolidated post-ceasefire access report would be issued from Amman on Friday.