US President Donald Trump has signed a decree saying the United States recognizes Israeli sovereignty over the Golan Heights.
The territory was seized from Syria by Israel in the 1967 Six-Day War.
Mr Trump signed the document at the start of a meeting with Israeli Prime Minister Benjamin Netanyahu.
The decree formalised Mr Trump's statement on 21 March saying it was time for the United States "to fully recognise" Israeli sovereignty over the Golan Heights.
The move appeared to give Mr Netanyahu a boost ahead of the closely contested 9 April Israeli elections.
"Your decision to recognise Israel's sovereignty on the Golan Heights is so historic," Mr Netanyahu told his US counterpart.
"Your recognition is a twofold act of historic justice. Israel won the Golan Heights in a just war of self-defence, and the Jewish people's roots in the Golan go back thousands of years," he said.
Mr Trump, who in 2017 took the step of recognising disputed Jerusalem as Israel's capital, called the Golan declaration "a long time in the making."
"It should have taken place many decades ago," he said.
Mr Netanyahu was visiting Washington for the conference of the American Israel Public Affairs Committee (AIPAC), the pro-Israel US lobby.
Russia warned that Trump's proclamation would set off a "new wave" of tensions in the Middle East, while Syria denounced the "blatant attack" on its sovereignty.
US allies Britain and France have said that they still consider the Golan Heights "Israeli-occupied" in line with UN resolutions, as experts warned of repercussions to the United States unilaterally recognising the results of military conquest.
Lebanon said the decision violates international law and undermines the prospects for peace.
In a statement carried by the NNA state news agency, the Lebanese foreign ministry said the move "violates all the rules of international law" and "undermines any effort to reach a just peace".
"The Golan Heights are Syrian Arab land, no decision can change this, and no country can revisit history by transferring ownership of land from one country to another," it said.
The ministry said attempts by Israel to expand its territory by way of "force and aggression" would only isolate the Jewish state.
Arab League Secretary-General Ahmed Aboul Gheit has condemned Mr Trump's recognition of the Golan Heights as Israeli territory, saying the decision does not change the area's status, according to a statement published by Egypt's state news agency MENA.
Israel annexed the Golan in 1981 but had won no international support.
The territory had long been seen as a comparatively easy problem to resolve, as both Syrian President Bashar al-Assad and the Golan's 20,000 settlers are largely secular in a region dominated by religious feuds.
The Tánaiste and Minister for Foreign Affairs said that Ireland regards the Golan Heights as Syrian territory under Israeli occupation.
Simon Coveney said the Government has no plans to alter that position.