Palestinian gunmen abducted two U.N. contractors, including one women demanding on the Palestinian Authority to put pressure on a rival family that seized his relative.
The two workers were not believed to be in any immediate danger, and Palestinian security officials said they were trying to secure their release.
United Nations Development Program officials identified the employees as Steve Sabila, a Palestinian photographer, and Zoe Constantine, a freelance writer from Australia. Both were working as consultants for the agency when they were seized Friday afternoon.
UNDP officials declined further comment, saying the situation was too sensitive.
Witnesses said the U.N. workers were taken from a car by a group of armed Palestinians in Gaza City. Shortly after, relatives of Palestinian security officer Jihad Abed said they were holding the pair. Abed was kidnapped on Thursday by Palestinian rivals and remained in captivity on Friday.
In a telephone interview, a relative of Abed described the U.N. workers as "guests" and said they were safe and drinking tea with the family. The family allowed cameramen into the home to see the hostages.
The relative, declining to be identified, said the family wants the Palestinian Authority to guarantee the safety and freedom of Abed.
Gen. Ala Hosni, the Palestinian police commander in Gaza, pledged to free the captives.
"We are not going to allow this incident to continue, and the kidnappers should release the hostages immediately," he said. "Our troops moved to the place where we believe the hostages are held, and they are working to guarantee their safety."
The Palestinian security forces have been weakened by more than four years of fighting with Israel, internal rivalries and corruption. As a result, Gaza suffers from frequent lawlessness, with vigilante gunmen operating in many areas with virtual impunity, the AP reports.