US security adviser Sullivan says Israel is acting in Syria for its own defence

U.S. national security adviser Jake Sullivan defended Israel's operations in Syria since the fall of President Bashar al-Assad, saying on Thursday it had a right to defend itself from risks to its security.

Since the shock collapse of Assad's government over the weekend, Israel has moved troops into the buffer zone on the Syrian side of the dividing line with the Israeli-occupied Golan Heights and has conducted hundreds of airstrikes to destroy Syrian army weapons and equipment.

"What Israel is doing is trying to identify potential threats, both conventional and weapons of mass destruction, that could threaten Israel, and, frankly threaten others as well," Sullivan told a press conference in Tel Aviv following a meeting with Prime Minister Benjamin Netanyahu.

In the aftermath of Assad's flight from Syria, Israeli jets and missile ships hit military targets including fighter jets, helicopters, naval vessels, missile stores and weapons manufacturing sites to stop them from falling into the hands of the rebel forces that toppled Assad.

Sullivan said the situation in Syria presented a range of risks "including the potential for fracture in that state". He added that power vacuums could give room for terrorist groups to grow and said the new power in Damascus could be hostile to neighbours including Israel.

"All of those are possibilities," he said.

Countries including France and the United Arab Emirates have condemned Israel's move into the buffer zone but Sullivan said the United States had "every expectation" the move would be temporary.

Israel has said its incursion into the buffer zone and its seizure of strategic areas of Mount Hermon, overlooking Damascus, was a temporary and limited measure to ensure its security.

Prime Minister Benjamin Netanyahu has said the collapse of Assad's government meant that no power was in place to enforce the agreement that created the zone following the 1973 Arab-Israeli war and Israel would remain only until a suitable arrangement was found.

But it remains unclear how long that situation will last.