United Nations officials said that health workers began vaccinating children in the northern Gaza Strip against polio on Tuesday, but noted that convoys carrying critical supplies of fuel and medicine were facing increasing obstruction and delays caused by Israeli forces.

The main U.N. aid agency operating in Gaza said that the Israeli military had detained a convoy of international and local staff members from various U.N. bodies at gunpoint for about eight hours on Monday as they traveled to northern Gaza to help roll out the polio vaccination campaign.

Stéphane Dujarric, the U.N. spokesperson, told reporters at a news briefing Tuesday that the conduct of Israeli forces had endangered the lives of the U.N. staff and ran contrary to mandated protections under international humanitarian law. He described a volatile encounter that unfolded after Israeli troops stopped the convoy at a checkpoint and said they wanted to hold two of its members for questioning.

“The situation escalated very quickly, with soldiers pointing their weapons directly towards our personnel in the convoy. The U.N. vehicles were encircled by Israeli forces and shots were fired,” he said. The convoy was then approached by Israeli tanks and a bulldozer, “which proceeded to ram the U.N. vehicles from the front and from the back, compacting the convoy with U.N. staff inside.”

Israeli forces continued to hold the convoy at gunpoint while senior U.N. officials attempted to de-escalate the situation with Israeli authorities, Dujarric said. The two staff members were interrogated and eventually released, and the convoy returned to U.N. bases without being able to complete its mission.

‘Palestinian suspects’

The Israeli military said in a statement that it had intelligence suggesting there were “Palestinian suspects” with the convoy but did not say what they were suspected of doing. In another statement Tuesday, it said that “Israeli security forces questioned the suspects in the field and then released them.”

The episode highlighted the challenges facing humanitarian efforts like the vaccination campaign and what U.N. officials say is increasing Israeli obstruction of aid deliveries to Gaza.

The United Nations delivered the first of two doses of oral vaccine to 446,000 Gazan children in the center and south of Gaza in two operations this month as part of a campaign negotiated with Israel to halt the quick-spreading polio virus. The global body aimed to vaccinate roughly 200,000 more children in the north of Gaza in the third phase, lasting through Thursday, and then to repeat the whole operation in a month’s time to deliver the second oral vaccine dose.

The Israeli military said the convoy halted on Monday was “for a U.N. personnel rotation.”

“Unfortunately, it’s not isolated,” WHO spokesperson Tarik Jasarevic said of the convoy’s detention, reporting that four attempts by the agency to deliver supplies to Shifa in the last four days had failed.

The Israeli military said it was looking into the claims.