Israel's aim to annex parts of the occupied West Bank was clearly "illegal", the UN's human rights chief said on Monday, warning that the consequences could be "disastrous".

Just days before Israel intends to kick-start plans to annex its West Bank settlements and the Jordan Valley, Michelle Bachelet added her voice to the chorus urging Prime Minister Benjamin Netanyahu to drop the proposal.

"Annexation is illegal. Period," the United Nations High Commissioner for Human Rights said in a statement.

"Any annexation. Whether it is 30 per cent of the West Bank, or five per cent," she said, urging Israel to "listen to its own former senior officials and generals, as well as to the multitude of voices around the world, warning it not to proceed along this dangerous path".

International condemnation of the possible Israeli annexations has mounted ahead of July 1, when Israel could take its first steps towards implementing part of a US-proposed Middle East peace plan.

US President Donald Trump's proposals, rejected outright by the Palestinians, pave the way for annexing key parts of the West Bank, including settlements long considered illegal by the majority of the international community.

'Shockwaves' 

Bachelet urged Israel to shift course, warning that "the shockwaves of annexation will last for decades, and will be extremely damaging to Israel, as well as to the Palestinians".

"The precise consequences of annexation cannot be predicted,” she said.

"But they are likely to be disastrous for the Palestinians, for Israel itself, and for the wider region.”

The UN rights chief warned that "any attempt to annex any part of the occupied Palestinian territory will not only seriously damage efforts to achieve lasting peace in the region, it is likely to entrench, perpetuate and further heighten serious human rights violations that have characterised the conflict for decades”.

Her statement cautioned that such a move would almost certainly lead to increased restrictions on Palestinians’ right to freedom of movement, as their population centres would become enclaves.

In addition, significant tracts of private land would likely be illegally expropriated, and even in cases where this does not occur, many Palestinians could lose access to cultivate their own lands.

And Palestinians who found themselves living inside the annexed areas would likely experience greater difficulty accessing essential services like healthcare and education, while humanitarian access could also be blocked.

Bachelet’s office warned that Palestinians inside the annexed area would come under heavy pressure to move out, pointing out that entire communities currently not recognised under Israeli planning would be at high risk of "forcible transfer”.

And settlements, which are already recognised as a clear violation of international law, would almost certainly expand.

"This is a highly combustible mix,” Bachelet said.

"I am deeply concerned that even the most minimalist form of annexation would lead to increased violence and loss of life, as walls are erected, security forces deployed and the two populations brought into closer proximity.”

The UN rights chief warned that "the existing two-tier system of law in the same territory will become embedded, with devastating impacts on the lives of Palestinians who have little or no access to legal remedy”.

Bachelet said illegal annexation would not change Israel’s obligations under international law as an occupying power towards the occupied population.

"Instead, it will grievously harm the prospect of a two-state solution, undercut the possibilities of a renewal of negotiations, and perpetuate the serious existing human rights and international humanitarian law violations we witness today.”