US President Donald Trump says it is time the US fully recognises Israel's sovereignty over the Golan Heights, which it captured from Syria in 1967.
In a tweet, Mr Trump declared that the plateau was of "critical strategic and security importance to the State of Israel and regional stability".
Golan Heights is a rocky plateau, covering about 1200 sq km about 60km south-west of Syrian capital Damascus. It has more than 30 Jewish settlements with an estimated 20,000 settlers, and there are also about 20,000 Syrians in the area, most of them members of the Druze sect.
Israel seized most of the Golan from Syria in the closing stages of the 1967 Middle East War, and thwarted a Syrian attempt to retake it in 1973. It applied its administration and law to the Golan in 1981, but other governments did not recognise the act.
There was no immediate response from Syria, which has sought to regain control of the region. The two countries remain technically in a state of war, and UN observers are deployed to monitor the 70km-long demilitarised zone.
Mr Trump's declaration comes Israeli Prime Minister Benjamin Netanyahu faces a closely fought general election on 9 April, as well as a series of possible corruption charges.
Mr Netanyahu, who has warned about the military "entrenchment" of his country's arch-enemy Iran in the Syria conflict, tweeted his thanks to Mr Trump on Thursday.
"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," he wrote.
Richard Haass, a former senior US state department official who is now president of the Council on Foreign Relations think-tank, said he "strongly disagreed" with Mr Trump's decision.
In a tweet, Mr Haass said the move violated a UN Security Council resolution, "which rules out acquiring territory by war".
In 2017, Mr Trump announced that the US recognised Jerusalem as Israel's capital, triggering angry reaction from the Palestinians.
Last year, the US moved its embassy from Tel Aviv to Jerusalem.
The Palestinians claim East Jerusalem as the capital of a future state, and according to the 1993 Israel-Palestinian peace accords, its final status is meant to be discussed in the latter stages of peace talks.
A formal announcement of the shift in US policy looks set to come when Mr Netanyahu visits Washington next week.
Like the Trump administration's decision to move the US embassy to Israel from Tel Aviv to Jerusalem, this will be seen as another demonstration of its unreserved support for Israel, and for Mr Netanyahu in particular. That will surprise no-one.