Israel has admitted – after mounting pressure – that its troops may have used white phosphorus shells in contravention of international law, during its three-week offensive in the Gaza Strip.

One of the places most seriously affected by the use of white phosphorus was the main UN compound in Gaza City, which was hit by three shells on 15 January. The same munition was used in a strike on the al-Quds hospital in Gaza City the same day.

Under review by Colonel Shai Alkalai is the use of white phosphorus by a reserve paratroop brigade in northern Israel.

According to army sources the brigade fired up to 20 phosphorus shells in a heavily built-up area around the Gaza township of Beit Lahiya, one of the worst hit areas of Gaza.

The internal inquiry – which the army says does not have the status of the full investigation demanded by human rights groups including Amnesty International and Human Rights Watch – follows weeks of fighting in which Israel either denied outright that it was using phosphorus-based weapons, or insisted that what weapons it was using "were in line with international law".