United Nations chief Antonio Guterres has called on Israel to abandon plans to annex parts of the occupied West Bank, saying such a move would be a "most serious violation of international law".

The UN secretary-general made the comments in a report to the Security Council on Tuesday, a day before the 15-member body holds its twice-yearly meeting on the Israeli-Palestinian conflict.

The government of Israeli Prime Minister Benjamin Netanyahu has said it could begin the annexation process from July 1.

In the document, Guterres said an Israeli annexation would be "devastating" for hopes of fresh negotiations and an eventual two-state solution.

"This would be calamitous for Palestinians, Israelis and the region," he said, adding that the plan threatened "efforts to advance regional peace".

Guterres's comments came a day after thousands of Palestinians protested in Jericho against the Israeli plans, in a rally also attended by dozens of foreign diplomats.

US President Donald Trump's controversial “peace” plan gave a green light for Israel to annex large swaths of the occupied West Bank, including settlements considered illegal under international law, and the Jordan Valley.

Unveiled in late January, Trump's plan proposed the establishment of a demilitarised Palestinian state on the remaining patchwork of disjointed parts of the Palestinian territories without occupied East Jerusalem. The plan has been rejected in its entirety by the Palestinians.

For decades, Israel has enjoyed bipartisan US support that enabled it to ignore international criticism and numerous UN resolutions over its occupation of Palestinian territories.

Earlier this month, hundreds of international law professors and scholars signed an open letter condemning the Israeli plans to annex territory in the West Bank, calling it a "flagrant violation of bedrock rules of international law, and would also pose a serious threat to international stability in a volatile region".