Trump said Thursday it was time for Washington to recognise Israel's sovereignty over the strategic territory, which it seized from Syria in the Six-Day War of 1967 and annexed in a move never recognised internationally.

His abrupt tweet triggered delight in Israel, but outrage from other countries in the region as well as powerhouses such as Russia and Turkey.

Moscow warned the policy U-turn could spark new conflicts.

AAP

"Such appeals can considerably destabilise an already tense situation in the Middle East," Kremlin spokesman Dmitry Peskov said.

"Hopefully it will remain (just) a call."

Any such move would break with UN Security Council resolutions and with more than half a century of US foreign policy, which treated the Golan as occupied territory whose future would be negotiated in talks with Syria on a comprehensive peace.

The territory's return has always been a key Syrian national demand, championed by government and rebels alike throughout the bloody civil war that has ripped the country apart since 2011.

In an angry retort, the Syrian government said Trump's comments disregarded international law.

"The American position towards Syria's occupied Golan Heights clearly reflects the United States' contempt for international legitimacy and its flagrant violation of international law," a foreign ministry source told the official SANA news agency.

Trump's comments showed the extent of his administration's "blind bias" towards Israel.

"The Golan was and will remain Arab and Syrian," the source said.

The foreign ministry sent a letter to UN Secretary General Antonio Guterres, urging him to reiterate the UN's rejection of Israeli claims over the Golan, SANA said.

'National commitment'

Turkey, which hosted the last indirect peace talks between Israel and the Syrian government in 2008 but has backed Syrian rebels, said the change risked plunging the region into a "new crisis".

"We will never allow the occupation of Golan Heights to be made legitimate," President Recep Tayyip Erdogan said.

Iranian Foreign Minister Mohammad Javad Zarif tweeted he was "shocked by @realDonaldTrump continuing to try to give what is not his to racist Israel."

And France added its voice to the chorus of outrage, saying the Golan had been "occupied by Israel since 1967" and it did not recognise Israel's annexation.

EPA

In his tweet, Trump said the Golan was "of critical strategic and security importance to the State of Israel and Regional Stability!"

"After 52 years it is time for the United States to fully recognise Israel's Sovereignty over the Golan Heights," he said.

The Arab League said Trump's comments were "completely outside international law".

The Gulf Cooperation Council said Trump's statement would not change the internationally recognised fact that the "Golan heights are Syrian lands forcefully occupied by Israel".

Jordan's Foreign Minister Ayman Safadi said lasting peace in the region requires Israel to withdraw from all Arab territories it occupies, including the Golan.

Egypt urged "respect (for) legitimate international resolutions and the United Nations Charter on the unacceptability of land appropriation by force".

Following decades of calm along the Golan armistice line after the Arab-Israeli conflict of 1973, tensions flared with the eruption of civil war in Syria in 2011.

Israel provided medical assistance to wounded rebel fighters and repeatedly struck government positions in response to stray fire across the frontier.

It has also targeted suspected positions of Iran and its Lebanese ally Hezbollah, which have intervened militarily to back President Bashar al-Assad.

Israeli thank you

Since the Syrian government decisively defeated rebel fighters near the armistice line last year with Iranian and Hezbollah support, Israel has repeatedly vowed to prevent its arch enemies from establishing a long-term military presence.

Israeli Prime Minister Benjamin Netanyahu, who is seeking re-election next month, swiftly thanked Trump for his announcement.

"At a time when Iran seeks to use Syria as a platform to destroy Israel, President Trump boldly recognises Israeli sovereignty over the Golan Heights," the right-wing prime minister wrote on Twitter.

Leon Panetta, a veteran Democrat who served as CIA director and defence secretary among other roles, blasted Trump for "tweeting out another policy that obviously has not been worked out with our international partners".

The Golan move is Trump's latest diplomatic bombshell as he seeks to redraw the fraught Middle East in Israel's favour.

In 2017, Trump went against decades of practice in recognising the disputed city of Jerusalem as Israel's capital, rather than the previously accepted Tel Aviv.