Israeli leaders have lauded Donald Trump for his list of decisions in support of their country since taking office, but the mercurial president's withdrawal of US troops from Syria will not rank among them.

After Trump's surprise announcement of the pullout last week, Israel is concerned over whether its main enemy Iran will have a freer hand to operate in the neighbouring country, analysts say.

Israel's response to the announcement has been measured — careful to point out that it respects the US decision, coupled with pledges to continue to defend its interests in Syria.

But beneath those public pronouncements are worries over whether Iran will seek to take advantage of the US absence from the war-torn country and if Russia will respond to Israel's calls to limit it.

Beyond that, the manner in which the decision was taken and announced — and US Defence Secretary Jim Mattis's resignation in response — may also give Israeli leaders pause, some analysts say.

“Since it's our major ally, we want the United States to be strong ... and we want an ally which is being perceived in the region as strong and effective,” said Eyal Zisser, vice rector of Tel Aviv University and who has written extensively on Syria.

“And I think that what worries some Israelis is what message does this decision -- the way it was taken, what stood behind it — send to the region?”

'Even expand our activities'

The United States has only around 2,000 troops in Syria focused on fighting the Islamic State group, but they have been deployed in two areas along the Iraqi border, helping keep Iranian movement into the country in check.

There have been warnings from Israel and others that Iran is seeking to form a “land bridge” across to the Mediterranean, and some analysts have said that the US withdrawal could help that effort.

With Iran supporting Syrian President Bashar al-Assad in his country's civil war, Israeli Prime Minister Benjamin Netanyahu has long pledged to keep it from entrenching itself militarily next door.

Israel has repeatedly taken action, carrying out hundreds of airstrikes in Syria against what it says are Iranian military targets and advanced arms deliveries to Hezbollah, the Iranian-backed Lebanese group.

With the United States pulling out, Israel may look more to Russia, which is also backing Assad, to use its influence to limit Iran, some analysts say.

But that is not a given, and a friendly fire incident in September that led to a Russian plane being downed by Syrian air defences during an Israeli strike remains an issue.

The incident angered Russia and complicated Israel's operations in Syria, particularly after Moscow's delivery of the advanced S-300 air defence system there in response.

Netanyahu and Israel's military chief of staff, Gadi Eisenkot, on Sunday sought to tamp down concerns over the withdrawal.

The Israeli premier has indicated he was not taken off-guard, saying he had spoken with Trump two days before the December 19 announcement as well as US Secretary of State Mike Pompeo the previous day.

“The decision to remove the 2,000 US soldiers from Syria won't change our consistent policy,” Netanyahu said Sunday.

“We will continue to act against Iran's attempt to establish a military presence in Syria, and if the need arises, we will even expand our activities there.”

'A free ride'

Yaakov Amidror, a former national security adviser to Netanyahu and ex-military intelligence official, noted US troops were not directly involved in Israel's fight against Iran's presence in Syria.

But he said concerns over whether Iran will take advantage of the US withdrawal were legitimate.

“From now on, it will be a free ride for the Iranians and they will use the corridor logistically to enhance their capabilities to build the military forces in Syria and to help Hezbollah afterwards,” he told AFP.

An analysis by the US-based Centre for Strategic and International Studies think tank said “Israel is among the most important losers” of the withdrawal, along with the United States' Kurdish allies in Syria.

But Netanyahu has vowed that Israel will continue to “defend ourselves” and Eisenkot, the military chief of staff, called it “a significant event but it should not be overstated”.

“For decades we've been handling this front alone,” said Eisenkot.