Activists feared Friday for the safety of a young Saudi woman they say was returned to the kingdom against her will, in a case highlighting tight restrictions on women.
The witness said Lasloom approached her while in transit at the airport in Manila, saying "airport officials had confiscated her passport and boarding pass" for a Sydney-bound flight.
The Canadian said she helped Lasloom film social media videos about her plight. In one of them she said: "If my family comes they will kill me," HRW said.
Arranged marriages are the norm in Saudi Arabia, where a "guardianship" system requires a male family member, usually the father, husband or brother, to grant permission for a woman's study, travel and other activities.
"Lasloom's whereabouts are currently unknown," HRW said in a statement from Manila.
The Canadian witness, who spent several hours with Lasloom at the airport in Manila, reported that two of Lasloom's uncles arrived, the New York-based watchdog said.
It also quoted an airline security official as saying he heard Lasloom "screaming and begging for help" on Tuesday before security personnel and men who appeared to be Middle Eastern carried her "with duct tape on her mouth, feet and hands" at the airport.
Asked about the HRW statement by AFP on Friday, the Philippine immigration department said it had held no one of Lasloom's name and no Saudi national.