The EU's top diplomat, Josep Borrell, urged Israel on Tuesday not to annex the Jordan Valley, a large swathe of the occupied West Bank, warning of Palestinian protests if it went ahead.
Israel captured the West Bank in a 1967 war. The Palestinian Authority wants to make the area part of a future state, but Israeli Prime Minister Benjamin Netanyahu announced his intention on Tuesday to annex the valley if he wins re-election.
"This may happen … You can be sure it's not going to be peaceful," Borrell told the European Parliament.
Around 65,000 Palestinians and 11,000 Israeli settlers live in the Jordan Valley and northern Dead Sea area, according to the Israeli human rights group B'Tselem. The main Palestinian city there is Jericho, with around 28 villages and smaller Bedouin communities.
Borrell, who travelled to Washington last week, also reiterated his rejection of parts of US President Donald Trump's new peace plan for the Middle East.
The US plan would give Israel most of what it has sought during decades of conflict, including nearly all Palestinian land on which it has built settlements.
"The proposals tabled two weeks ago clearly challenge the internationally agreed parameters. It is difficult to see how this initiative can bring both parties back to the table," Borrell said of Israel and the Palestinians.
"I made this point to my (US) interlocutors: we need to ask ourselves whether this plan provides a basis for progress or not."