A "watershed moment" that will constitute a "most serious violation of international law" - that is how United Nations chief Antonio Guterres has described Israel's plans to annex parts of the occupied West Bank and Jordan Valley.

Addressing a virtual meeting of the UN Security Council on Wednesday, Guterres repeated a call for Israel to drop its United States-backed plans, which could be put in motion as soon as next week.

If implemented, the UN secretary-general said, annexation would "grievously harm the prospect of a two-state solution and undercut the possibilities of a renewal of negotiations".

"I call on the Israeli government to abandon its annexation plans."

Guterres's call was echoed by other leaders and foreign ministers addressing the virtual meeting, warning that Israel's unilateral action could trigger a major escalation in the region.

"For three decades, real peace and the creation of an independent Palestinian state, remain elusive ... Despair is dominating the Palestinian mood and scene,"  Arab League chief Ahmed Aboul Gheit said.
Israel set to annex one-third of occupied West Bank (2:57)

Wednesday's meeting is seen as the last international one before Israeli Prime Minister Benjamin Netanyahu's right-wing government starts planned discussions on July 1 over the annexation of the West Bank, territory Israel captured in a 1967 war and that Palestinian leaders seek for a future state.

At the UNSC meeting, Nickolay Mladenov, the UN peace envoy for the Middle East, warned of the legal, security and economic implications of such a move.

"Annexation could irrevocably alter the relation of Israeli-Palestinian relations. It risks upending a quarter of a century of international efforts and support of a future viable Palestinian state," Mladenov said.

"Today, we are further than ever from this goal [of a two-state-solution]."