The Israel Defense Forces is investigating whether a reserve paratroops brigade made improper use of phosphorus shells during the fighting in Gaza.

The brigade fired about 20 such shells in a built-up area of northern Gaza.

Aside from this one case, the shells were used very sparingly and, in the army's view, in compliance with international law.

The IDF's use of phosphorus shells has sparked great criticism both in Israel and in the international media. The army therefore appointed Col. Shai Alkalai, an artillery officer, to investigate the issue, and his probe is still in progress.

According to senior army officers, the IDF used two phosphorus-based weapons in Gaza. One, the sources said, actually contains almost no phosphorus. These are simple smoke bombs - 155mm artillery shells - with a trace of phosphorus to ignite them.

Alkalai's probe is thus focusing on the second type: phosphorus shells, either 81mm or 120mm, that are fired from mortar guns. About 200 such shells were fired during the recent fighting, and of these, according to the probe's initial findings, almost 180 were fired at orchards in which gunmen and rocket-launching crews were taking cover.

The one problematic incident was the reserve paratroops brigade that fired about 20 such shells in a built-up area of Beit Lahiya. Many international organizations say phosphorus shells should not be used in heavily populated areas. The brigade's officers, however, say the shells were fired only at places that had been positively identified as sources of enemy fire.

The 120mm shells, a recent acquisition, have a computerized targeting system attached to a GPS. Brigade commanders say they were very effective, but they were also responsible for two very serious mishaps: a strike on a UNRWA school that killed 42 Palestinians and a friendly fire incident that seriously wounded two officers.