The International Committee of the Red Cross has condemned Israel's building of a barrier in the West Bank as "contrary" to international law.
Israel says the barrier is designed to stop suicide bombers
The aid agency said the barrier, whose proposed route cuts into Palestinian areas, went "far beyond what is permissible for an occupying power".
Israel says the barrier is designed to stop suicide bombers.
But Palestinians dispute the barrier's legality and say the wall is little more than a land grab.
The ICRC's comments come just days before a hearing at the International Court of Justice in The Hague on the legality of the controversial barrier.
Israel says the court does not have the authority to hold hearings on the barrier.
The ICRC, a neutral, Swiss-based organisation, has been observing construction of the security fence.
It says that, where the barrier runs through occupied territory, thousands of Palestinians have been deprived of access to water, health care and education.
It has also caused extensive damage to Palestinian land and property, the agency adds.
"The problems affecting the Palestinian population in their daily lives clearly demonstrate that it [the barrier] runs counter to Israel's obligation under [international humanitarian law] to ensure the humane treatment and well-being of the civilian population living under its occupation," it said in a statement.
"The ICRC, therefore, calls upon Israel not to plan, construct or maintain this barrier within occupied territory."
The organisation stressed that it respected Israel's right to protect itself and the statement did not call for a halt to building altogether.
But BBC's Imogen Foulkes in Geneva says it was an unusually outspoken statement from a normally neutral body.
The Israeli ambassador to international organisations in Geneva, Yaakov Levy, told the Associated Press news agency that Israel "regretted" the ICRC's decision to criticise the barrier.
"There is a danger that the position presented by the ICRC will be turned into a political tool against Israel's measures of self-defence," he said.