Donald Trump's declaration that it is "time to recognise" Israel's sovereignty over the Golan Heights has been described as "illegal", "irresponsible" and a "direct violation of United Nations" decisions.
The president's announcement signalled a shift in US policy and comes ahead of the Israeli prime minister's planned visit to the White House next week.
However, it has been met with condemnation from around the region - most notably from Syria, who Israel seized the area from during the 1967 Middle East war, annexing it in 1981.
A foreign ministry statement said that Mr Trump's comments confirm "the blind bias of the United States to the Zionist entity", referring to Israel.
It added the policy shift will not change "the fact that the Golan was and will remain Arab and Syrian".
Russia's foreign ministry said in a statement that a change in the status of the Golan Heights would be a "direct violation of United Nations decisions".
A spokesman for Iran's foreign ministry, quoted on state TV, said: "This illegal and unacceptable recognition does not change the fact that it belongs to Syria."
The US president made his announcement on Twitter, writing: "After 52 years it is time for the United States to fully recognise Israel's Sovereignty over the Golan Heights, which is of critical strategic and security importance to the State of Israel and Regional Stability!"
The Golan is internationally recognised as occupied territory and Syria has demanded its return as a condition in the event of a future peace agreement.
Mr Trump's tweet was posted during his secretary of state Mike Pompeo's visit to Jerusalem, although Mr Pompeo declined to comment on the president's remarks.
Israeli Prime Minister Benjamin Netanyahu thanked the US president for "boldly" recognising Israel's control over the territory.
He tweeted: "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. Thank you President Trump!"
This cannot be undone and I am sure it will be greatly appreciated.
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