Israel says Palestinian terrorists are exploiting the United Nations Relief and Works Agency, UNRWA, even as it admitted that military experts might have mistaken a stretcher for a rocket in a video showing an object being loaded into a U.N. vehicle.

The latest allegations come following the news that the Israeli Army has arrested 13 United Nations employees on suspicion of terrorist activities. Army officials say charges will be brought against the 13, but will not provide specifics. The men were arrested after Israel said it had evidence that a U.N. ambulance was being used to transport a rocket to be used on attacks against Israel.

The army now says there is a difference of opinion among analysts as to whether it was a rocket or a stretcher, as the U.N. officials here have maintained.

Tuesday night, foreign ministry official Gideon Meir repeated Israel's charge that terrorists are using U.N. facilities. He says Israel has concrete evidence terrorists are using U.N. ambulances and, in some cases, shooting out of hospitals.

He says the evidence will be presented to a U.N. team due to arrive in the region for an already scheduled trip. The team has been tasked with investigating Israeli claims.

U.N. Secretary-General Kofi Annan launched an investigation into those accusations, late Monday. The United Nations insists it does not hire terrorists and does not tolerate any militant activity of any kind by its staff.

Meanwhile, Israeli military operations in the Gaza Strip entered a seventh day, Wednesday. Palestinian witnesses say three Palestinians were killed and 10 children were injured when an Israeli tank fired several times in the northern Gaza town, Beit Lahiya. A 15-year-old boy was reported to have been killed in one house and two men in another. The shelling took place just before dawn. Hospital officials say 10 children were injured, several of them seriously, when a tank fired on the house where they were sleeping.

The Israeli military says it fired on a group of gunmen who had launched an anti-tank rocket from a house in the neighborhood.

The commander of the militant group Islamic Jihad in Gaza was killed in an Israeli missile attack near Gaza City, Tuesday. Three members of the militant Al Aqsa Martyrs Brigades were killed in a separate missile attack in the Jabalya refugee camp. A 13-year-old girl was shot by Israeli troops, in what the army describes as a regrettable incident.

Israel launched its offensive following a Palestinian rocket attack a week ago that killed two Israeli children.