An 11-year-old Palestinian schoolgirl was shot in the stomach and critically wounded by Israeli gunfire Tuesday as she sat inside a United Nations (search) school in a Gaza refugee camp, U.N. officials said.

The incident threatened to escalate tensions between Israel and the United Nations Relief and Works Agency (search), which runs the school.

Last month, a 10-year-old girl was killed by Israeli gunfire while sitting at her desk at the same school.

Israeli troops frequently clash with Palestinian gunmen throughout the Gaza Strip (search).

UNRWA, the main U.N. agency for Palestinian refugees, has accused Israeli troops of firing indiscriminately in crowded areas. Israel accuses the militants of using populated areas for cover and endangering the lives of civilians.

In the aftermath of another incident. Israel's top military prosecutor opened an investigation into a platoon commander soldiers accuse of emptying an ammunition clip into a 13-year-old Palestinian girl to make sure she was dead, the army said Monday.

In Tuesday's shooting, UNRWA said troops fired two shots from the nearby Gush Katif bloc of Jewish settlements, one of which hit Ghadeer Imkhmar, a fifth-grade student at a U.N. elementary school in Khan Younis (search).

Palestinian hospital officials said the girl was in critical but stable condition after emergency surgery.

Israeli military sources said Palestinian militants had fired mortar shells at an army outpost in the area and soldiers returned fire. The army was investigating the reported shooting of the schoolgirl.

UNRWA said it was the fourth shooting of a student at one of its schools in Gaza in the past two years.

Israeli-U.N. relations have been strained for years, with Israel repeatedly accusing the world body of bias.

A U.N. team is in Israel to investigate Israeli contentions that a U.N. Palestinian driver loaded a rocket into a U.N. ambulance during recent fighting in northern Gaza.

Israel has backed away from its claim. But officials have said they want the U.N. team to determine whether the world body employs people who "aid and abet" Palestinian militants.

The issue of the 13-year-old girl who was killed surfaced when soldiers who serve under the platoon commander — whose name was not made public — told Israeli newspapers and TV reporters about the incident.

Iyman Hams, 13, was shot and killed Oct. 5. Initially the army said soldiers shot and killed the girl as she planted a bomb near an army outpost in southern Gaza.

But after soldiers recounted a different version of events to the media, an investigation was launched. The soldiers told the Yediot Ahronot newspaper they were unhappy the platoon commander had not been suspended.

Dozens of Palestinian children have been killed since fighting erupted four years ago, but the army rarely launches investigations into the incidents.