United Nations Secretary-General Antonio Guterres on Monday urged Israel to "halt and reverse" its decision last week to build hundreds of new settler homes in the West Bank.
Israel on Sunday approved the construction of 780 homes in the West Bank, ordered last Monday by Prime Minister Benjamin Netanyahu.
The decision is "a major obstacle to the achievement of the two-State solution, and a just, lasting and comprehensive peace" in the Middle East, Guterres said in a statement.
"The establishment by Israel of settlements in the Palestinian territory occupied since 1967, including East Jerusalem, has no legal validity and constitutes a flagrant violation under international law.
"Settlement expansion... further erodes the possibility of ending the occupation and establishing a contiguous and viable sovereign Palestinian State, based on the pre-1967 lines," Guterres said.
The move has also been condemned by the European Union, the United Kingdom, Israeli opposition members and settlement watchdog Peace Now, among others.
The UK said Monday that it was "seriously concerned" by the government's decision, asking it to end the project "immediately."
"The UK is seriously concerned by the Government of Israel's decision to approve the construction of 780 new settlement units across the Occupied Palestinian Territories," a Foreign Commonwealth and Development Office spokesperson said.
"Settlements are illegal under international law and risk undermining the physical viability of the two-state solution," the spokesperson said. "We call for the construction of these in East Jerusalem and elsewhere in the West Bank to cease immediately."
Peace Now said that over 90% of the homes lay deep inside the West Bank, which the Palestinians seek as the heartland of a future independent state, and over 200 homes were located in unauthorized outposts that the government had decided to legalize.
Israeli settlements in the West Bank are regarded as illegal by much of the international community.
There are currently some 450,000 Israeli settlers in the West Bank, living amid an estimated 2.8 million Palestinians.