Peter Herby's comments on the 17th of January on the ICRC's homepage:
"If ICRC delegates in the field gather credible and precise evidence of violations, or if ICRC medical personnel corroborate reports by others, the ICRC would begin by discussing this with the party concerned – rather than speaking publicly – in keeping with our standard practices. We have not commented publicly on the legality of the current use of phosphorous weapons by Israel, contrary to what has been attributed to us in recent media reports."Within the article linked to above, he also outlines the the rules of international humanitarian law applicable to the use of phosphorous weapons and intended to spare civilians.
Peter Herby is Coordinator of the Mines-Arms Unit in the Legal Division of the International Committee of the Red Cross.
The Human Rights Watch (Kenneth Roth, "The Incendiary IDF", 22 January, 2009) and Amnesty International ( The conflict in Gaza; A briefing on applicable law, investigations, and accountability, January 2009) are more forthright in their discussion of the illegality.
2nd April, 2009: Also refer here.