EU countries ought to draw "red lines" for US president Donald Trump on the Arab-Israeli conflict, Herman Van Rompuy, the former head of the EU Council, has said.

"Trump's plan is shaping up to be a major test of Europe's resolve to defend its interests and values in the face of increasing pressure on the multilateral, law-based order we hold so dear," Van Rompuy told EUobserver in a statement.

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Donald Trump recognised Israeli annexation of East Jerusalem, including sacred Muslim sites (Photo: Hadar)

"Europe should lay down its red lines and make clear it will oppose any proposal that crosses them," he added.

The US president is expected to shortly unveil his plan on how to end the oldest conflict in Europe's neighbourhood.

The EU and UN back a two-state solution, with East Jerusalem as Israel and Palestine's shared capital.

But Trump has already recognised Israel's annexation of Jerusalem from Palestine and of the Golan Heights from Syria, amid EU concern that he is preparing to also recognise its annexation of settlements in Palestine's West Bank.

Van Rompuy is a former Belgian prime minister who chaired EU summits for five years from 2009 to 2014 and who works as an adviser to the European People's Party, a centre-right political bloc in Europe.

He spoke out after adding his name to an open letter on the conflict by 37 former EU foreign ministers and leaders sent before the Easter holiday.

Israel was "sliding into a one-state reality of unequal rights" and Trump was "gambling with the security and stability" of Europe's neighbourhood, the letter, sent to the EU Council and the foreign service, said.

It urged the 28 EU countries to restate their commitment to Palestinian statehood in order to protect their own "vital interests" as well as broader principles of international diplomacy.

Trump's "disturbing" actions had made the EU restatement "more important than ever," Mogens Lykketoft, a Danish MP and former foreign minister who wrote and organised the open letter, told EUobserver at the time.

Van Rompuy added that "scaling up Europe's collective efforts to safeguard agreed parameters and secure respect for international law is the only way forward towards a lasting solution to the [Arab-Israeli] conflict".

The US and EU are Nato allies with decades of history of shared Middle East and wider foreign and economic policy.

But Trump has also questioned America's Nato membership, torn up other EU and UN-backed accords, such as one on Iran nuclear arms control, and imposed protectionist tariffs on EU trade.

The array of 37 names included former ministers from France, Germany, the Netherlands, Nordic countries, and the UK.

It also included ones from Italy and Spain in the south and from Austria, Poland, and Romania in the east.

But the Czech Republic, Hungary, Poland, and Slovakia have forged closer relations with Trump and Israel in recent years, undermining European unity on the Middle East.

It "always worked better" when Europe worked as one with the US, Van Rompuy added.

"However, in situations where fundamental differences emerge, we must not shirk our own commitment to defending the global rules-based order," he said.