Israel announced the move on Sunday, the army saying the step stemmed from political decisions taken after the June killing of three Israeli teenagers snatched from a roadside in the same area, known to Israelis as Gush Etzion settlement bloc.
"The Secretary-General is alarmed by yesterday's announcement by Israeli authorities to declare as so-called 'state land' nearly 1,000 acres of land in the Bethlehem area of the West Bank," the spokesman said.
"The secretary-general calls on Israel to heed the calls of the international community to refrain from settlement activity and abide by its commitments under international law and the Quartet Road Map."
The United States urged Israel earlier Monday to reverse the plan, which has angered the Palestinians and alarmed Israeli peace campaigners.