Secretary-General Ban Ki-moon is appealing for the release of all missing and detained United Nations staff and urging that perpetrators responsible for killing a U.N. Development Program official in Iraq and others be brought to justice.

In Syria alone, the U.N. said 35 staff members are missing or detained, the majority from the U.N. agency for Palestinian refugees.

The U.N. chief said in a message marking the International Day of Solidarity with Detained and Missing Staff on Friday that member states are holding five U.N. personnel "without any reasons given for the arrests."

"Personnel, especially those deployed under dangerous conditions, deserve full protection and rights," the secretary-general said. "I call on all parties to respect the rights, privileges and immunities of United Nations personnel."