The United Nations told its staff in Yemen to stay home on Monday, a day after gunmen shot and killed a German embassy guard. International humanitarian organizations also have advised staff to limit their movements. “Staff should remain at home and not travel. No travel to shops or elsewhere is approved. Tomorrow is uncertain, and all staff should be very cautious,” the U.N. Department of Safety and Security said in an internal note Sunday. The German security guard was killed in Hadda district, in southwestern Sanaa, where the embassy is located.