Israel has carried out hundreds of air strikes in neighbouring Syria against what it says are Iranian targets.

It accuses its arch-enemy Tehran, which like Moscow supports the Syrian regime in the seven-year civil conflict, of seeking to entrench itself militarily in the country.

"In the past Russia has been permissive in consultation with the Israelis about Israeli strikes against Iranian targets inside Syria," US envoy James Jeffrey said.

"We certainly hope that that permissive approach will continue," he told journalists in a conference call.

His comments came after Syrian regime forces accidentally downed a Russian plane during an Israeli air strike over Syria in September.

Moscow blamed the incident, which killed 15 Russians, on the Israeli plane using the larger Russian one for cover, which Israel disputed.

Following the incident, Russia sent advanced air defence missiles to Damascus.

But Israel's Prime Minister Benjamin Netanyahu last month told Russia his country must continue to hit hostile targets in Syria to prevent Iran from establishing a military presence across the border.

"We understand the existential interest and we support Israel," Jeffrey said.

Under President Donald Trump, the United States has withdrawn from a landmark deal with Iran over its nuclear programme and reimposed crippling sanctions on the Islamic republic.

President Bashar al-Assad's forces have regained large swathes of Syria from rebels and jihadists since 2015.

In October, US Secretary of State Mike Pompeo told a pro-Israel group Washington would refuse any post-war reconstruction assistance to Syria if Iran was present.

Syria's war has killed more than 360,000 people and displaced millions more since starting in 2011.