At least 25 Palestinian U.N. workers have been detained by Israel for as long as two years under charges Israel refuses to reveal, U.N. officials said Wednesday.

The statements coincided with the arrival of a U.N. team in Jerusalem to investigate Israeli charges that a Palestinian employee of the U.N. Relief and Works Agency loaded a Qassam rocket into a U.N. ambulance during fighting in the northern Gaza Strip.

Israel has backed away from its claim that footage from a drone plane showed an agency driver loading a rocket into the ambulance. But Israel will demand that the investigators determine whether the world body employs people who "aid and abet" Palestinian militant groups, an Israeli official said.

Israeli-U.N. relations have been strained for years, with Israel repeatedly accusing the world body of bias. The United Nations, meanwhile, has criticized Israeli policies and military operations in Palestinian areas. Relations have become more strained because of Israel's eight-day incursion into Gaza.

The issue of the detentions surfaced Tuesday in Jerusalem when an Israeli army commander said that 13 agency employees had been arrested during an unspecified period for links to militant groups and that some would be charged soon.

Fred Eckhard, the U.N. spokesman, said Wednesday that "one staffer has been detained in Gaza for over two years and is currently awaiting trial, and 24 are being held under administrative detention without charge or trial." In each case, Israel would not explain the reason for the detention, he said.